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9004 



Bureau of Mines Information Circular/1985 



The Bureau of Mines Noise-Control 
Research Program— A 10-Year Review 



By William W. Aljoe, Thomas G. Bobick, 

Gerald W. Redmond, and Roy C. Bartholomae 




UNITED STATES DEPARTMENT OF THE INTERIOR 



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Information Circular 9004 

The Bureau of Mines Noise-Control 
Research Program— A 10-Year Review 



By William W. Aljoe, Thomas G. Bobick, 

Gerald W. Redmond, and Roy C. Bartholomae 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Model, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 







Library of Congress Cataloging in Publication Data; 



The Bureau of Mines noise-control research program. 

(Information circular / United States Department of the Interior, Bu- 
reau of Mines ; 9004) 

Bibliography: p. 83-85. 

Supt. of Docs, no.: I 28.27:9004. 

I. Mineral industries— Noise control. I. Aljoe, William VV. II. 
United States, Bureau of Mines. III. Series: Information circular 
(United States. Bureau of Mines) ; 9004. 



-^Wf^tSOM — -fTD893.6.M5] 622s [622'. 8] 84-600226 



a CONTENTS 

C^ Page 



^ Abstract 1 

^ Introduction 1 

Part 1. — Government Involvement In mining noise control 2 

The mining noise problem. 2 

K) Federal noise regulations pertaining to mining 2 

^ The Bureau of Mines approach to mining noise control 5 

^ Part 2. — Overview of the Bureau's noise-control research 6 

^ Underground coal mining 7 

^s^ Coal extraction methods 8 

.> Haulage methods 10 

Roof support methods 11 

Underground hardrock mining 12 

Hardrock extraction methods 12 

Haulage methods 15 

Roof support methods 16 

Surface mining 16 

Preparation and processing plants 17 

Part 3. — Results of selected research programs 19 

Underground coal mining 19 

Coal cutting 19 

Chain conveyors 29 

Mantrip vehicles 34 

Stoper drills 36 

Underground hardrock mining 41 

Jumbo-mounted percussion drills 42 

Load-haul-dump machines 48 

Surface mining 53 

Bulldozers 55 

Front-end loaders 58 

Preparation and process ing plants 61 

Coal preparation plants 62 

Taconlte processing plants 67 

Nonmetallic processing plants 72 

Additional research on screen-noise abatement 73 

Use of hearing protectors in the mining environment 74 

Limitations of hearing protector effectiveness 75 

Assessing earmuf f attenuation 76 

Hearing protector interference with required acoustical cues 77 

Part 4. — Future Bureau noise-control efforts 77 

Facilities and equipment 78 

Research programs 79 

Coal cutting 79 

Conveying 79 

'^ Percussion drilling. 79 

'\^ Hearing protectors 80 

^ Technology transfer 80 

Summary 80 

References 83 

(\ 



11 



ILLUSTRATIONS 



Page 



1 . Hearing thresholds of retired miners 2 

2 . Hearing loss among miners 3 

3. Noise levels and operating times of underground coal mining machines 7 

4 . Noise sources on continuous mining machines 8 

5. Noise sources on typical coal mine stoper drill 12 

6. Major components of a jumbo drill rig 13 

7. Noise sources on jumbo-mounted drills 14 

8. Noise sources of mobile, diesel-powered surface mining equipment 17 

9 . Grinding mills in taconite processing plant 18 

10. Linear cutting apparatus 20 

11. Coal-cutting force versus time 21 

12. Coal-cutting force (power spectral density) versus frequency 22 

13. Continuous miner in reverberation room 25 

14. Cutting sequence of auger-type continuous miner 26 

15. Standard auger-miner cutting head 27 

16. Reduced-noise auger-miner cutting head 27 

1 7 . Fabrication drawing of reduced-noise auger 28 

18. Noise-producing components of a continuous miner chain conveyor 29 

19. Noise-control treatments on conveyor decks and sidewalls 30 

20. Noise-control treatments on conveyor idler (tail) roller and takeup plate. 31 

21. Mantrip vehicle used in retrofit noise-control program 35 

22. Mantrip noise sources and transmission paths 35 

23. Resilient wheels to isolate mantrip from wheel-rail noise 36 

24. Redesigned suspension system of factory-quieted mantrip 37 

25. Motor enclosure of factory-quieted mantrip 37 

26. Wraparound jacket-type muffler for stoper drill 38 

27. Damping collar for stoper drill steel 39 

28. Stoper with retrofit noise-control treatments in operation 39 

29. Internal components of standard stoper drill with rifle-bar rotation 39 

30. Internal components of redesigned "quiet" stoper drill 40 

3 1 . Redesigned "quiet" stoper on f eedleg 41 

32. Drilling controls of redesigned "quiet" stoper drill 42 

33. Jumbo drill within retrofit muffler enclosure (cover open) 43 

34. Jumbo drill within retrofit muffler enclosure (cover closed) 43 

35. Schematic views of retrofit muffler enclosure for jumbo drill 44 

36. Retrofit shroud tube for controlling jumbo drill-steel noise 44 

37. Jumbo drill with retrofit noise-control treatments in underground zinc 

mine 45 

38. Redesigned, noise-controlled jumbo drill at start of hole 47 

39. Redesigned, noise-controlled jumbo drill at completion of hole 47 

40. Noise sources on typical diesel-powered LHD vehicle 49 

41. Sound-absorbing foam lining within LHD transmission compartment 49 

42. Retrofit acoustical enclosure for LHD engine 50 

43. Retrofit noise-control treatments on LHD engine cooling fan 51 

44. Exhaust muffler on redesigned, noise-controlled LHD vehicle 52 

45. LHD operator-compartment noise-control treatments 53 

46. Vibration-isolation mount for LHD transmission 53 

47. Caterpillar D-9G bulldozer (ROPS-FOPS only) with retrofit noise-control 

treatments 54 

48. Noise-control treatments installed on Caterpillar D-9G bulldozer (ROPS- 

FOPS only) 54 



ILLUSTRATIONS—Continued 



iii 



Page 



49. Step-by-step noise reduction of Caterpillar D-9G bulldozer (ROPS-FOPS 

only) 55 

50. Noise-control treatments on cab-equipped Caterpillar D-9G bulldozer 57 

51. Step-by-step noise reduction of cab-equipped Caterpillar D-9G bulldozer... 58 

52. Noise-control treatments installed on International Harvester TD-25C 

bulldozer 59 

53. Step-by-step noise reduction of International Harvester TD-25C bulldozer.. 59 

54. Noise-control treatments installed on Caterpillar 988 front-end loader.... 60 

55. Step-by-step noise reduction of Caterpillar 988 front-end loader 61 

56. Noise-control treatments installed on International Harvester H-400 B 

front-end loader 61 

57. Step-by-step noise reduction of International Harvester H-400 B front-end 

loader 62 

58. Flow chart of Georgetown coal preparation plant 62 

59. Curtain around screening area in Georgetown coal preparation plant 63 

60. Schematic of enclosed gallery-type walkway in preparation plant 69 

61. Secondary crusher area in taconite processing plant 70 

62. Taconite grinding mill and noise barrier 71 

63. Noise-control treatments for rapper on taconite fines screen 72 

64. Flow chart of screening simulation algorithm 74 

65. Noise pathways to ear protected by hearing protective device 75 

66. Quieted versus unquieted noise levels of underground mining equipment 81 

67. Quieted versus unquieted noise levels of preparation and processing plant 

equipment 82 

TABLES 

1. Typical noise levels of various sounds , 3 

2. Allowable noise exposure time per day 4 

3. Illustrative calculation of worker NEI 4 

4. "Noise offenders" in surface mining operations 16 

5. Results of continuous miner cutting-noise tests 24 

6. Summary of aboveground chain conveyor noise-control tests 32 

7 . Summary of underground chain conveyor noise-control tests 33 

8. Results of retrofit jumbo drill noise-control tests 45 

9 . Breakdown of noise sources on unmodified LHD vehicle 48 

10. Results of underground tests of redesigned, noise-controlled LHD 53 

11. Breakdown of bulldozers used in U.S. surface mines, 1977, by model 55 

12. Summary of bulldozer retrofit noise-control treatment results 56 

13. Summary of treatments and costs for bulldozer noise-control treatments.... 56 

14. Summary of front-end loader retrofit noise-control treatment results 58 

15. Summary of treatments and costs and for front-end loader noise-control 

packages 60 

16. Short-term and long-term effectiveness of noise-control treatments at 

Georgetown coal preparation plant 64 

17. Noise-control treatments used at Georgetown coal preparation plant 65 

18. Noise-control alternatives for new coal preparation equipment 68 

19. Noise-control treatments installed in three nonmetallic processing plants. 73 





UNIT OF MEASURE ABBREVIATIONS 


USED IN THIS 


REPORT 


dB 


decibel 


lb/ft2 


pound per square foot 


dBA 


decibel, A-weighted 


kHz 


kilohertz 


dB/Hz 


decibel-second per cycle 


mi/h 


mile per hour 


ft 


foot 


min 


minute 


ff Ibf 


foot pound (force) 


ym 


micrometer 


ff Ibf/min 


foot pound (force) per minute 


ms 


millisecond 


ga 


gauge 


pet 


percent 


h 


hour 


re.v/niin 


revolution per minute 


h/d 


hour per day 


s 


second 


Hz 


hertz 


ton/h 


ton per hour 


in 


inch 


yd3 


cubic yard 


in/s 


inch per second 


yr 


year 


Ibf 


pound (force) 







THE BUREAU OF MINES NOISE-CONTROL RESEARCH PROGRAM-A 10-YEAR REVIEW 

By William W. Aljoe, ^ Thomas G. Bobick, ^ Gerald W. Redmond, and Roy C. Bartholomae"^ 



ABSTRACT 

This report summarizes the Bureau of Mines noise-control research 
program from 1972 to 1982, Each segment of the mining industry — under- 
ground coal, underground hardrock, surface mining, and processing 
plants — has different noise-control problems because of vast differ- 
ences in working procedures, equipment, and workplace design. The Bu- 
reau has identified the most serious noise problems in each segment and 
has developed strategies for attacking these problems. 

This publication points out the need for noise control in the mining 
industry, discusses Federal regulations governing worker exposure to 
noise, and describes the Bureau's overall approach to mining noise- 
control research. It traces the history of noise overexposure in each 
segment of the mining industry and discusses the major noise sources. 
It provides detailed information on noise-control research efforts in 
the Bureau's major areas of emphasis, including the results of these 
efforts. Finally, the report discusses the Bureau's future role in 
research on mining noise control, emphasizing the need to expend more 
effort on long term in-house investigations into the noise problems 
that have been identified in past programs as the most serious ones, 

INTRODUCTION 

Noise exposures of workers in the mining industry often exceed those 
specified under Federal noise regulations. Because of the severity and 
diversity of this problem, the Bureau of Mines has conducted a wide 
variety of noise-control research efforts. This report summarizes the 
Bureau's noise-control research program during the 1972-82 period, giv- 
ing an overall picture of the Bureau's activities in this area and its 
major accomplishments. Further details on the projects described in 
this publication can be found in the reports included in the reference 
list or by contacting the authors of this report at the Bureau's Pitts- 
burgh Research Center, 



^Mining engineer, 
^Industrial hygienist. 
^Supervisory electrical engineer. 
Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA, 



PART 1. — GOVERNMENT INVOLVEMENT IN MINING NOISE CONTROL 



THE MINING NOISE PROBLEM 

Noise is often regarded merely as a 
nuisance; however, it is also a wide- 
spread occupational health problem. In 
the mining industry, the noise problem is 
especially serious because overexposure 
can cause permanent hearing loss. The 
extent and severity of the noise problem 
in mining was revealed in a 1976 study by 
the National Institute for Occupational 
Safety and Health (NIOSH) (25).^ This 
study found that underground coal miners 
had measurably worse hearing than the na- 
tional average population; for example, 
the median hearing threshold of retired 
miners was 20 dB greater than that of the 
general population (fig. 1). Figure 2 
shows that, at age 60, over 70 pet of all 
miners had a hearing loss greater than 25 
dB , and about 28 pet had a hearing loss 

'^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



greater than 40 dB, Miners experience 
greater hearing impairment than most 
other industrial workers because of their 
work-related overexposure to noise. 
Because this hearing loss occurs gradual- 
ly over many years, the individual is not 
aware of it until he or she notices dif- 
ficulties in coimnuni eating with other 
people or an inability to hear safety 
signals in the workplace, 

FEDERAL NOISE REGULATIONS 
PERTAINING TO MINING 

The Federal Government regulates the 
noise exposure of miners under the Fed- 
eral Mine Safety and Health Act of 1977 
(Public Law 95-164). This act, which 
supersedes the previous Federal Coal Mine 
Health and Safety Act of 1969 (Public Law 
91-173) , covers surface and underground 
operations of metal and nonmetal mines as 
well as coal mines. The Code of Federal 
Regulations (CFR) defines permissible 
noise exposures for all mine workers in 








J L 



4 5 



FIGURE 



1 1 1 

_ Ages 


1 




65 to 74 


^ 


^ 


— / 




^^( 


_ J 


^O" 


_ 


/ / 
— 1 1 
/ / 
/ / 
/ / 




- 


/ / 
/ / 

ir^ p' 




- 








'■-o' 






1 1 1 


1 






6— --0 



/ KEY 

General 

population 

Retired - 

miners 

I I I 



I 234501 2345 
FREQUENCY, kHz 

Hearing thresholds of retired miners. 



Q. 

{/f 
(/) 
O 

_J 

o 
z 
a: 

X 
X 

I- 



DC 
LlI 



80 



60- 



40 



20 



Hearing loss >25 dB 








20 30 40 50 

MINERS' AGE, yr 

FIGURE 2. - Hearing loss among miners. 



terms of noise dose, which includes both 
the level and duration of the noise. ^ 

^Mining noise regulations are contained 
in the following sections of 30 CFR: 
55.5-50 (surface metal and nonmetal 
mines), 56.5-50 (sand, gravel, and 
crushed stone), 57.5-50 (underground met- 
al and nonmetal), 70.500 through 70.511 
(underground coal), and 71.800 through 
71.805 (surface coal). 



Noise level is measured in deci- 
bels,^ using a sound level meter, and the 
A-weighted decibel (dBA) is the unit of 
measure used in the regulations. The 
A-weighting scale takes into account the 
fact that the human ear is more respon- 
sive to high-frequency sounds (1,000 to 
5,000 Hz) than to low-frequency sounds 
(below 1,000 Hz). Table 1 shows the typ- 
ical A-weighted noise levels of everyday 
events. 

Table 2 gives the relationship between 
noise level and allowable exposure time 
per day for mining operations. Exposure 

^The decibel is a dimensionless rela- 
tionship between two quantities, de- 
fined as 20 times the logarithm of 
the ratio of a measured quantity to a 
reference quantity. When measuring 
noise, the sound pressure is the quan- 
tity of interest, and the sound pres- 
sure level (SPL) is defined as 
measured sound pressure 
reference sound pressure 
The standard reference sound pressure 
used in the United States is 4.32 x 10"^ 
Ib/ft^. Further discussion of the SPL 
can be found in reference 18 or in any 
standard textbook on noise control. 



20 log,o 



-^ dB. 



TABLE 1, - Typical noise levels of various sounds, A-weighted decibels 



Noise source 



Noise level 


Subjective e 


140 


Deafening. 


'130 


Do. 


120 


Intolerable. 


110 


Do. 


105 


Very loud. 


100 


Do. 


90 


Do. 


80 


Loud, 


70 


Do. 


60 


Moderate. 


50 


qvilet. 


40 


Faint. 


30 


Do. 


20 


Very faint. 


10 


Do. 



Jet engine 

Pneumatic chipping 

Underground pneumatic percussion drill, , 
Automatic punch press; hand grinding..,. 
Bulldozer (at operator's position)...... 

Auto horn (at 10 ft) 

Construction site in urban area 

Busy street; school cafeteria 

Loud radio; stenographic room 

Restaurant; department store 

Average office; quiet conversation 

Residential area at night 

Quiet residence (inside) 

Background in TV studio 

Rustle of leaves 



Threshold of pain. 



TABLE 2. - Allowable noise exposure 
time per day 



90. 


No 


ise level, 
dBA 


Allowable exposure 
time , h 

8 


c>?. 








6 


95. 
P7. 








4 
3 


100. 








2 


102. 








1.5 


lOS. 








1 


107, 








.75 


110. 








.5 


115. 








<.25 



to noise levels less than 90 dBA is not 
regulated. It is evident from table 2 
that the higher the noise level, the 
shorter the allowable exposure time is. 
At 90 dBA, the maximum allowable exposure 
time is 8 h/d; every 5-dBA increase in 
noise level reduces the maximum allowable 
exposure time by half. Exposure to con- 
tinuous noise levels higher than 115 dBA 
is not permitted by law. 

In practice, compliance with noise reg- 
ulations is determined by measuring each 
worker's noise exposure index (NEI) , 
which includes both the noise level and 
the exposure time at each level. NEI is 
defined as 

Ci C2 

— L + _^ + • • • » 

Ti T2 ^ 

where C^, C2 , etc., are the actual times 
the worker was exposed to each noise lev- 
el above 90 dBA, and T] , T2 , etc., are 



the allowable exposure times (table 2) 
for the noise levels to which the worker 
was exposed. A worker is overexposed, or 
out of compliance, when the NEI value ex- 
ceeds 1. NEI values are often expressed 
as a percent of the allowable noise dose, 
i.e., actual NEI value x 100 pet. For 
example, table 3 shows that during an 8-h 
shift, a worker is exposed to noise for 
2 h at 89 dBA, 2 h at 90 dBA, 2 h at 95 
dBA, and 2 h at 100 dBA. The NEI for 
this worker is 



2 2 2 

I" + -| + ^ = 0.25 + 0.5 + 



1.0 



= 1.75 X 100 pet = 175 pet. 

Thus, the worker is out of compliance 
with Federal noise regulations (because 
the NEI exceeds 100 pet). 

The NEI measures only the worker's ex- 
posure to continuous noise; it does not 
include "impulse" or "impact" noise, 
which takes place over too short a time 
to be included in NEI calculations. Im- 
pulse or impact noise is defined as a 
sound that (1) reaches its peak level 
within 35 ms after initiation and (2) de- 
creases to at least 20 dB below its peak 
level within 500 ms after reaching the 
peak level (535 ms after initiation) 
(24). If the impulses recur at intervals 
less than 1 s apart, the noise is classi- 
fied as continuous. All of the Bureau's 
noise-control research efforts have dealt 
with continuous noise because this is by 
far the most prevalent type of mining 
noise. 



TABLE 3. - Illustrative calculation of worker NEI 
during 8-h shift 



Noise level. 


dBA 


Allowable 

exposure time 

(T), h 


Actual 
exposure time 
(C), h 


Dosage 
[(C/T) X 100], 
pet 


89 


Unlimited 

8 

4 

2 

NAp 


2 
2 
2 
2 

NAp 





90 


25 


95 


50 


100 


100 


Total 


1175 



NAp Not applicable, 



NEI. 



THE BUREAU OF MINES APPROACH 
TO MINING NOISE CONTROL 

Because noise affects large numbers of 
workers in all segments of the mining in- 
dustry the Bureau has become involved in 
many different aspects of noise control. 
The objective of the Bureau's noise- 
control research efforts is to investi- 
gate techniques that would, if adopted, 
help bring the mining industry into com- 
pliance with Federal noise regulations. 

Because the mining noise problem is so 
widespread, the Bureau must define those 
areas in which it can appropriately be 
involved. Three criteria must be met be- 
fore the Bureau will investigate a spe- 
cific noise problem: (1) The problem 
must result in serious worker overexpo- 
sure and risk of hearing loss; (2) it 
must affect a large number of mine work- 
ers; and (3) the mining industry itself 
must be incapable of solving the prob- 
lem because of insufficient financial re- 
sources or technical expertise. For ex- 
ample, the Bureau is conducting long-term 
basic studies into all the major noise- 
generating mechanisms of the coal-cutting 
process; this requires a level of effort 
and technical expertise that equipment 
manufacturers cannot provide. Converse- 
ly , the Bureau is not heavily involved in 
acoustical booth technology because these 
structures are commercially available and 
can be improved through efforts by the 
private sector. However, the Bureau has 
become involved in specific areas where 
acoustical booth technology is especially 
difficult to apply, such as in portable 
mineral processing plants. 

An important aspect of the Bureau's ap- 
proach to noise control is its emphasis 
on engineering controls for the major 
noise sources in the mining workplace. 
Although Federal noise regulations spec- 
ify the use of both engineering and ad- 
ministrative (e.g., job switching) con- 
trols as the primary means of addressing 
mining noise problems , Bureau research 
has shown that engineering controls can 
often be more effective. For example, 
job switching during any given day would 
be nearly impossible in many mines 



because of long, nonproductive travel 
times between noisy and quiet workplaces. 
In addition, serious safety hazards could 
result if inexperienced workers were 
switched into noisy jobs that require 
high levels of skill. Finally, if an 
engineering control can be applied suc- 
cessfully to the noise source, admini- 
strative controls will be unnecessary 
and the solution will generally be more 
permanent. 

At first glance, personal hearing pro- 
tectors (earplugs, earmuffs, etc.) seem 
to be a relatively cheap, simple solution 
to almost any noise problem. However, 
Federal regulations state that hearing 
protectors should be considered as only a 
secondary noise-control treatment in sit- 
uations where engineering or administra- 
tive controls cannot be used. Reasons 
for this include the following: (1) Ear- 
plugs and earmuffs do not usually provide 
the same degree of protection in the min- 
ing workplace as they do in the labora- 
tory or in other types of workplaces; 
(2) miners often refuse to wear hearing 
protectors because they are uncomfort- 
able, annoying, or cumbersome (especially 
earmuffs); and (3) hearing protectors can 
prevent miners from hearing warning sig- 
nals such as "roof talk" (the noise that 
often precedes a roof fall) or backup 
alarms on moving equipment. Although the 
Bureau is still conducting research into 
the potential effectiveness of hearing 
protectors in the underground mining 
environment, they are viewed as only a 
temporary solution to a problem that 
requires the use of engineering controls. 

Unfortunately, the mining environment 
often prohibits the use of standard engi- 
neering noise-control techniques common 
to other industrial workplaces. In most 
cases (especially underground) , the min- 
ing workplace is very confined, and it 
changes constantly because it is con- 
trolled primarily by local geology rather 
than by engineering design. Physical 
conditions are usually quite severe 
(e.g., wet floors and sloughing roof and 
ribs), thus restricting the use of stan- 
dard acoustical materials such as sound- 
absorbing foam panels. Another reason 



treating the mining workplace would be 
impractical is that many miles of mine 
tunnels would have to be treated. Sur- 
rounding the noise source with an acous- 
tical enclosure is another standard en- 
gineering noise-control technique that 
cannot usually be employed because the 
most severe mining noise sources are of- 
ten large mobile machines such as contin- 
uous miners and jumbo drills. 

Engineering noise controls in mining 
can be as simple as adding sound-absorb- 
ing material to the inside of a bulldozer 
operator's cab, or as complex and demand- 
ing as redesigning the inner workings of 
percussion drills to produce less noise 
while maintaining the same drilling 
power. The two basic engineering noise- 
control strategies are (1) to install 
noise-control treatments on existing min- 
ing equipment (the retrofit approach) and 
(2) to design inherently quieter mining 
equipment. 

The retrofit approach is usually 
preferred if a straightforward noise- 
control technique can be employed, and it 
is especially desirable in the capital- 
intensive mining industry. If a retrofit 
noise-control treatment is successful, it 
can often allow timely and relatively 
inexpensive compliance with Federal noise 
regulations. An acoustical cab for a 
bulldozer operator is one example of a 
successful application of the retrofit 
approach. However, even in this simple 
case, careful application of the noise- 
control treatments is essential for 
optimum noise reduction. Unfortunately, 
the retrofit approach is not appropri- 
ate for many existing pieces of mining 
equipment. 



In some cases , engineering noise con- 
trols can be incorporated into the ma- 
chine design without affecting overall 
performance. For example, the manufac- 
turer of an underground load-haul-dump 
(LHD) vehicle can easily install vibra- 
tion-isolation mountings (a noise-reduc- 
ing feature) on the engine and trans- 
mission of a newly built machine. Even 
though this treatment requires only minor 
dimensional changes in the original vehi- 
cle design, it could not be accomplished 
through the retrofit approach. The rede- 
sign approach is especially advantageous 
when the manufacturer has already decided 
to change its equipment for production 
puropses; noise-control technology would 
then constitute only a fraction of the 
redesign cost. 

Cost-effectiveness is of primary impor- 
tance in any approach to mining noise 
control. The Bureau considers an engi- 
neering noise-control technique to be 
cost-effective if it (1) does not reduce 
production or productivity, (2) costs 
relatively little to install, (3) does 
not increase long-term maintenance costs, 
and (4) does not introduce new hazards 
into the mining environment. The total 
amount of noise reduction is also impor- 
tant to the cost-effectiveness of any 
noise-control program. That is, a low- 
cost "quick-fix" approach may not be 
the best way to treat a noisy machine 
if a more expensive long-term research 
and development program can result in a 
much quieter, more efficient machine 
design. The Bureau has consistently 
sought the most cost-effective engineer- 
ing approaches to mining noise problems 
and has therefore used both retrofit and 
redesign techniques in its research. 



PART 2. —OVERVIEW OF THE BUREAU'S NOISE-CONTROL RESEARCH 



Historically, the amount of mechaniza- 
tion in mines has determined the extent 
of worker overexposure to noise. This 
part of the report reviews the history of 
mechanization in the four major segments 
of the mining industry (underground coal, 
underground hardrock, surface mining, and 
mineral processing) and identifies the 



most serious "noise offenders" in each 
segment. The Bureau research programs 
designed to control noise from these 
"noise offenders" are described briefly 
in the following sections; part 3 dis- 
cusses in detail some of the more suc- 
cessful Bureau research programs. 



UNDERGROUND COAL MINING 

When the first commercial U.S. under- 
ground coal mines were developed in the 
18th century, the three major unit opera- 
tions — extraction, haulage, and roof sup- 
port — were performed manually. The coal 
was cut from the solid bed using hand 
tools such as picks and bars, then shov- 
eled into baskets, boxes, carts, or 
wheelbarrows and dragged by workers or 
animals to the outside or to the foot of 
a shaft. Roof supports were emplaced 
with hand tools. Obviously, none of 
these manual extraction, hauling, and 
roof support methods generated much 
noise. 



longwall shearer, and the air-powered 
"stoper" roof drill — became possible. 
These three machines commonly produce 
noise levels that result in violations of 
Federal noise regulations (5), 

Figure 3 shows the noise levels and 
operating times of most of the noise- 
producing machines in today's underground 
coal mines. This illustration shows that 
at least one noise offender is present in 
each major unit operation (extraction, 
haulage, and roof support). Therefore, 
the Bureau has conducted research pro- 
grams in all three of these areas . 



Underground coal mining operations 
started to become mechanized in the late 
1800' s with the development of punching 
machines and chain-type cutters to under- 
mine the coal seam before blasting. Oth- 
er early mining machines were coal and 
rock drills, electric and compressed-air 
locomotives, and gathering-arm-type load- 
ing machines. Still, very few workers 
were overexposed to noise because few 
mining operations were mechanized. 

Full mechanization, and the noise asso- 
ciated with it, emerged in the 1920' s 
when gathering-arm-type loading machines 
became more common. Rubber-tired and 
crawler-mounted machines for cutting 
and hauling coal away from the face re- 
duced the number of workers underground, 
but those who remained were exposed to 
increasing amounts of machine-generated 
noise. The desire for increased coal 
production led to the development of 
larger, faster-moving machines with 
greater mechanical power; unfortunately, 
the noise associated with these machines 
also increased tremendously. 

The final development contributing to 
increased noise in the coal mining envi- 
ronment came just after World War II, 
when the introduction of the tungsten 
carbide cutting bit enabled coal mining 
machinery to cut both rock and coal. 
With the tungsten carbide cutting bit, 
the three noisiest machines in present- 
day coal mines — the continuous miner, the 



I25| r 



CD 
T3 



> 

LlI 



UJ 

to 




40 80 120 160 

OPERATING TIME, mm 



200 



FIGURE 3o - Noise levels and operating times of 
underground coal mining machines. 



Coal Extraction Methods 

Continuous Mining 

Figure 4 Illustrates five different 
noise-producing mechanisms on continuous 
mining machines: 

1. Face-radiated noise - When the bits 
on the rotating cutting drums strike and 
break the coal or rock, noise radiates 
from the coal or rock face, 

2. Cutting head noise - Cutting bit 
vibration is transferred to the drums, 
the boom holding the drums, and the rest 
of the machine structure. These vibrat- 
ing components often generate a very com- 
plex noise pattern. 

3. Gathering head noise - After the 
coal or rock is cut from the face, it 
falls onto a pan called the gather- 
ing head, beneath the cutting drums. 



Rotating arms or discs on this pan force 
the broken material into the mouth of a 
conveyor that carries it to the rear of 
the miner. Gathering head noise results 
from various impacts involving the ro- 
tating arms or discs, the pan, and the 
broken material. 

4. Conveyor noise - The conveyor con- 
sists of a continuously moving chain to 
which a series of flights are attached. 
The chain and flights move at the bottom 
of a metal trough. Scraping and impacts 
involving the chain, flights, trough, and 
broken material cause a great deal of 
noise. 

5. Motor and drive-train noise - The 
motors and drive train supplying power to 
the cutting drums, gathering head, con- 
veyor, and other machine components gen- 
erate noise both internally (gear noise) 
and externally (vibration Imparted to ma- 
chine structures). 




Motor and 
drive train noise 



I 



FIGURE 4. - Noise sources on continuous mining machines. 



Laboratory research sponsored by the 
Bureau (see part 3) has indicated that 
face-radiated noise could be reduced, 
without harming production, if the bits 
on the miner cutting head moved at a 
slower speed while penetrating deeper in- 
to the coal face (J^, 3), Unfortunately, 
present continuous miner designs are not 
adaptable to the Bureau's slow, deep- 
cutting technique. More research is 
needed to evaluate the potential noise 
reduction that can be obtained using 
slow, deep-cutting miners. 

Cutting head noise can be reduced by 
preventing the transfer of cutting bit 
vibration to the cutting drums and to 
other machine components. The Bureau is 
presently investigating the merits of 
completely redesigning the cutting drum 
of the miner to reduce vibration trans- 
fer. Preliminary laboratory studies have 
shown a potential for significant noise 
reduction, but full-scale tests with a 
continuous miner equipped with an "iso- 
lated cutting drum" are needed to verify 
the laboratory predictions. It appears 
that this approach (isolating the cutting 
drum) may be applicable to a wide vari- 
ety of coal- and rock-cutting machines 
because vibrational energy is absorbed 
close to its source — the interface be- 
tween the cutting bits and the coal or 
rock. 

Cutting head noise can also be reduced 
by limiting the vibration of the cutting 
drums and other structural components of 
the continuous miner. In one Bureau 
project (27) , the noise radiated from the 
cutting heads of a low-coal auger-type 
miner was reduced by about 10 dBA by en- 
larging the auger core, stiffening the 
helix surrounding the core, and damping 
its vibration with sand. (This project 
is discussed in detail in part 3.) Mine 
operators can easily modify their augers 
in the same manner with the aid of de- 
tailed fabrication instructions provided 
by the Bureau (28). This reduced-noise 
auger can also be obtained from the auger 
manufacturer. 

When the continuous miner is cutting 
and loading coal, the gathering head is 



usually less noisy than the cutting head 
or the conveyor. Although the impact of 
the rotating gathering arms on the pan 
generates significant noise when the 
miner is "running empty," this is not one 
of its major operating modes. Therefore, 
the Bureau has directed most of its ef- 
forts toward controlling cutting noise 
and conveyor noise rather than gathering 
head noise. Although disc-type gathering 
devices may be quieter than gathering 
arms, because they eliminate arm-to-pan 
impacts, no data exist to verify their 
potential for reducing of gathering head 
noise. 

Conveyor noise can be reduced by 
(1) smoothing discontinuities between 
various sections of the trough, (2) iso- 
lating the chain and flights from the 
trough, and (3) structurally damping the 
large metal panels of the trough. A re- 
cent Bureau project ( 16 ) showed that con- 
veyor-generated noise could be reduced 
by about 10.5 dBA through these tech- 
niques. However, this work was done in 
an aboveground test facility, using only 
the conveyor portion of the continuous 
miner (no cutting heads, gathering arms, 
drive train and motors, etc.). (For more 
details, see part 3.) Further testing on 
operating continuous miners is necessary 
to determine the overall noise reduction 
and durability of conveyor noise-control 
treatments. 



Motor and drive 
ally not as loud 
or noise, but it 
components of the 
orate. This noi 
negligible if sys 
tors and shorter, 
were used. Bureau 
directed toward 
source. 



train noise is usu- 
as cutting and convey- 
tends to increase as 
drive system deteri- 
se would probably be 
terns with smaller mo- 
simpler drive trains 
research has not been 
control of this noise 



Conventional Mining 

Although the number of coal mines using 
the conventional mining system has de- 
creased since the advent of the continu- 
ous miner, many smaller mines continue to 
use this system. In conventional mining, 
coal is blasted rather than cut from the 



10 



face, and mechanized coal extraction 
procedures include (1) undercutting the 
face with chain-type cutting machines, 

(2) drilling holes for explosives with 
machine-mounted coal drills, and 

(3) loading the coal into shuttle cars 
with gathering-arm-type loading machines. 

As indicated in figure 3, the loading 
machine (or loader) is the noisiest ma- 
chine used in conventional mines. Since 
loader noise is due primarily to conveyor 
noise and gathering head noise, control 
of loader noise includes the same 
conveyor-quieting techniques as have been 
described for continuous miners. In 
fact, controlling loader noise is some- 
what simpler than controlling miner noise 
because there is no coal-cutting noise. 

The Bureau has not conducted noise-con- 
trol research directed specifically to- 
ward conventional coal mining equipment, 
except for research on chain conveyors. 
The two major reasons for this are 
(1) the decreasing number of conventional 
mines as compared to continuous and long- 
wall mines and (2) the relatively low 
noise levels of coal cutters and face 
drills compared with the noise levels of 
continuous miners and longwall shearers 
(fig. 3). 

Longwall Mining 

Although longwall mining systems were 
almost nonexistent in the United States 
prior to 1965, they now produce about 
10 pet of all domestic coal mined un- 
derground. A continued increase in the 
use of these systems is expected in the 
future; therefore, the Bureau is now 
attempting to identify and develop the 
fundamentals that will be necessary to 
control all major noise sources in long- 
wall mining. Figure 3 shows that the 
longwall shearer operator is often ex- 
posed to more noise than the continuous 
miner operator. Approaches to longwall 
noise-control have been somewhat similar 
to those for continuous miners because 
both mining systems utilize rotary cut- 
ting heads and chain conveyors. Part 3 
describes the basic coal-cutting and 



chain conveyor research applicable to 
both continuous and longwall mining sys- 
tems. However, design differences be- 
tween the actual hardware in the two sys- 
tems have necessitated separate Bureau 
efforts in both areas (42-43) , 

Haulage Methods 

Because more coal miners work in the 
face area than in any other single loca- 
tion, the Bureau has concentrated on 
reducing the noise produced by face 
equipment. However, underground coal- 
haulage equipment also generates substan- 
tial noise, and the Bureau has addressed 
some of the problems in this area. 

Face Haulage 

Continuous miners and loading machines 
load coal into shuttle cars or some type 
of continuous face-haulage system. Shut- 
tle car noise is usually not as loud as 
continuous miner and loader noise (fig. 
3) , so the shuttle car operator is ex- 
posed to the most noise while loading 
coal. Although operators may also exper- 
ience noise levels slightly above 90 dBA 
while unloading the shuttle car, they 
usually spend more time tramming than 
loading and unloading combined. Since 
tramming is a relatively quiet procedure 
(less than 90 dBA) , overexposure of shut- 
tle car operators is minimal. 

Conveyors are undoubtedly the greatest 
source of face-haulage noise. Continuous 
coal-haulage systems in the face area 
usually contain one or more conveyors, or 
"bridges," mounted on a self-propelled 
carrier vehicle. The bridge carrier op- 
erator either rides on this vehicle or 
walks and/or crawls beside it while load- 
ing coal. Conveyor noise-control treat- 
ments developed for continuous miners 
could also be used to benefit bridge car- 
rier operators. 

Secondary and Main Mine Haulage 

Shuttle cars and continuous face-haul- 
age systems usually transfer coal to 
belt conveyors or locomotive-drawn rail 



11 



cars that haul the coal out of the mine. 
Electrically driven motors are the pri- 
mary noise sources in belt-conveyor sys- 
tems , but they do not usually contribute 
to worker overexposure. Likewise, the 
Mine Safety and Health Administration 
(MSHA) has not identified a serious over- 
exposure problem among operators of elec- 
trically powered haulage locomotives. 

Personnel Haulage 

In the early days of coal mining, when 
coal was mined and transported by hand or 
animals, men usually walked or crawled to 
the face areas. When electrical power 
was introduced into the mines , personnel- 
haulage vehicles were developed. Typical 
of these is the rail-mounted "mantrip" or 
"portal bus" that carries workers from 
the mine portal to the face areas. 

Because mantrips operate for only 15 to 
45 min per shift, they are not a primary 
cause of noise overexposure; however, 
they commonly produce noise levels rang- 
ing from 90 to 95 dBA and can contribute 
significantly to the total daily noise 
dose experienced by face workers. Man- 
trips generate noise primarily through 
(1) interaction between the wheels and 
rails and (2) operation of the electric 
motor and drive train. 

Bureau research programs aimed at con- 
trolling mantrip noise have been quite 
successful. (See part 3.) Noise levels 
have been reduced to below 90 dBA on two 
different mantrip vehicles. First, a 
mantrip was equipped with retrofit noise- 
control treatments on its wheels, passen- 
ger compartments, and motor enclosure 
(17) . Similar noise-control techniques 
were then used by a mantrip manufacturer 
to produce an "inherently quieter" vehi- 
cle at its factory (15) . 

Roof Support Methods 

Wooden posts, crossbars, and cribs were 
the original forms of coal mine roof sup- 
port and were usually emplaced by hand. 
Roof bolts, the most common means of roof 
support in present-day coal mines, were 



first used extensively in U.S. coal mines 
in the mid-1940' s. Holes must be drilled 
into the mine roof to install roof bolts, 
and two basic methods are used to drill 
roof bolt holes — handheld pneumatic per- 
cussion ("s toper") drills and machine- 
mounted electric rotary drills. 

Stoper drills are by far the loudest 
machines used in coal mines today, com- 
monly producing noise levels of about 120 
dBA (fig. 3). Because Federal regula- 
tions do not allow continuous noise lev- 
els above 115 dBA, any continuous opera- 
tion of a stoper drill would cause the 
operator to be overexposed. For this 
reason, the reduction of stoper drill 
noise was one of the Bureau's first high- 
priority areas of noise-control research. 
Part 3 includes a detailed description 
of the Bureau's stoper noise-control 
program. 

The noise-generating mechanisms of 
stoper drills are basically the same as 
those of jumbo-mounted pneumatic percus- 
sion drills used in hardrock mines. (The 
operating mode of jumbo drills is dis- 
cussed in some detail under the heading 
"Hardrock Extraction Methods" in the next 
section.) Although stopers are smaller 
and lighter than jumbo drills, the three 
major noise sources are the same — drill- 
body vibration, drill-steel vibration, 
and air-exhaust noise. Figure 5 shows a 
typical stoper drill and the location of 
these noise sources. 

Two basic approaches to stoper noise 
control were investigated — retrofit tech- 
niques and complete redesign of the 
stoper drill. Although the redesign ap- 
proach was more difficult and expensive 
than the retrofit approach, it resulted 
in a quieter, more efficient drilling 
machine. However, the retrofit treat- 
ments were also effective because they 
reduced noise substantially and did not 
require modification of the drill itself. 
The retrofit treatments reduced noise 
levels at the operator's position ranged 
from 102 to 106 dBA, reflecting an 11- to 
14-dBA reduction versus noise levels of 
untreated stopers (38). The redesigned 



12 



Driil steel 




FIGURE 5c. = Noise sources on typical coal 
mine stoper drill. 



about 90 to 95 dBA; this occurs while 
drilling the roof and tightening the 
bolt. However, these two operations com- 
prise only 55 pet of the time spent 
by the bolter in the face area and only 
20 pet of an 8-h shift. Based on these 
data, MSHA estimated that only 5 pet of 
all rotary roof bolter operators would 
be out of compliance with Federal noise 
regulations (_5 ) . For this reason, noise- 
control programs for rotary roof bolters 
have not been initiated by the Bureau. 

UNDERGROUND HARDROCK MINING 

Extraction, haulage, and roof support 
are unit operations that are common to 
both coal and hardrock mines. However, 
underground hardrock mining systems and 
equipment are quite different from those 
used in coal mines for the following rea- 
sons: (1) In most cases, explosives must 
be used for hardrock extraction because 
the rock is much too hard for continuous- 
type mining and cutting machines. (Ex- 
ceptions are "soft" ores such as salt, 
potash, and trona.) (2) Hardrock ore 
bodies are more irregular than coal seams 
and require more complex mine layouts . 
(3) Diesel-powered equipment is more 
prevalent in hardrock mines than in coal 
mines. 

Bureau studies in the mid-1970 's ( 26 ) 
identified the major noise sources in un- 
derground hardrock mines , and research 
since that time has included efforts to 
control these sources. Because under- 
ground hardrock mines use such a wide 
variety of equipment types , Bureau re- 
search has addressed only the most seri- 
ous noise offenders in these mines. 

Hardrock Extraction Methods 



("quiet") stoper reduced operator noise 
levels to about 98 dBA, reflecting a 22- 
dBA reduction in noise versus that of 
standard drills (13). A smaller, lighter 
version of the "quiet stoper" produced 
noise levels of 102 to 105 dBA and was 
more readily accepted by drill operators. 

Figure 3 shows that rotary roof-bolting 
machines can produce noise levels from 



Since the 16th century, hardrock ores 
have been freed from the earth by blast- 
ing. For more efficient blasting, holes 
are drilled into the rock mass, and 
explosives are placed in the holes. Al- 
though the types of explosives and drill- 
ing techniques used have changed sub- 
stantially through the years, the basic 
principles of drilling and blasting have 
not. Increased noise associated with 



13 



underground ore extraction is due primar- 
ily to the increased power of modern 
drilling machines. 

Early miners found that the most effec- 
tive way to bore a hole in rock was to 
place a hard, pointed object against the 
rock surface and strike the object with a 
hammer. At first this was done manually, 
but attempts were made throughout the 
1800' s to mechanize the process. The 
first mechanically powered rock drills 
were steam-driven; but they weighed sev- 
eral thousand pounds and were very cum- 
bersome, so they were unacceptable com- 
mercially. Compressed air proved to be a 
better power source; in 1861, the Mont 
Cenis Tunnel in the French Alps marked 
the first successful commercial applica- 
tion of pneumatic rock drills. Improve- 
ments in pneumatic rock drill design 
continued throughout the late 1800' s, in- 
cluding John George Leyner's development 
of a self-rotated striking tool (drill 
steel) with a hollow core to allow the 
passage of air or water for flushing the 
hole. 

Metallurgical improvements in the 20th 
century have permitted the development of 
high-strength rock drill components that 
impart tremendous amounts of energy to 
the rock face. Modern drills commonly 
deliver 30 blows per second to the face 



at about 200 ft'lbf per blow — approxi- 
mately 360,000 ft»lbf/min. 

Often, percussion drills are most ef- 
fective when mounted on a self-propelled 
vehicle called a jumbo. Figure 6 shows 
the major components of a typical jumbo 
drill system. The jumbo vehicle supports 
from one to three hydraulically powered 
booms that position the drill against the 
rock face. In operation, the drifter 
(drill body) generates a series of impul- 
sive blows upon the steel by means of 
a rapidly oscillating piston, which is 
driven by either a pneumatic or hydraulic 
power source. With each blow, a stress 
wave moves from the drifter through the 
steel and bit into the rock, which shat- 
ters under the tungsten carbide cutting 
edges of the bit. After each blow, the 
steel and bit rotate slightly to bring 
the cutting edges of the bit into contact 
with fresh rock surface. The feed, a 
chain- or screw-drive mechanism within a 
channel, supports the drifter and moves 
it forward as drilling progresses; and 
this forces the bit against the rock. 
The centralizer wraps around the drill 
steel and keeps it from wandering side- 
ways at the start of drilling. The chips 
and dust produced during drilling are 
flushed from the hole by either com- 
pressed air or water that flows through 
the center of the steel and out through 



Jumbo 




FIGURE 6. - Major components of a jumbo drill rig. 



14 



ports In the bit. The drill operator 
stands on a platform on the jumbo vehi- 
cle, just behind the booms. 

Figure 7 shows the noise-generating 
mechanisms of jumbo-mounted pneumatic 
percussion drills. These mechanisms can 
be placed in two broad categories: ex- 
haust noise and vibration of mechanical 
components. Exhaust noise, drifter-body 
vibration, and drill-steel vibration out- 
weigh the other noise sources shown 
in figure 7; therefore, the Bureau has 
focused its efforts on ways to control 
these sources. 

A straightforward approach for con- 
trolling exhaust noise is to channel it 
away from the operator through ductwork. 
A second approach is to attach a baffle- 
type muffler directly to the exhaust 
port(s) of the drifter. A third is to 
place the drifter inside an acoustical 
enclosure. These approaches are effec- 
tive in reducing noise, but all share a 
common problem: freezing. The rapidly 
expanding exhaust air cools quickly, and 
the moisture in the air condenses on the 
inside surfaces of the ductwork, muffler, 



or enclosure; after a short time, perhaps 
only a few minutes, ice begins to accumu- 
late, inhibiting the flow of exhaust air 
and causing the drill to stall. The Bu- 
reau presently has an in-house test set- 
up designed to investigate the muffler- 
freezing problem in more detail. Also, 
as described in part 3, ice-inhibiting 
enclosures for two different jumbo drills 
were developed under two recent Bureau 
contracts. The enclosures reduced both 
the exhaust noise and the noise generated 
by vibration of the drifter body. 

The vibrating drill steel often gener- 
ates more noise than any other component 
of the jumbo rig. To address this prob- 
lem, the Bureau is investigating several 
different drill-steel noise-control tech- 
niques. The two contracts mentioned 
above explored the "shroud-tube" concept, 
whereby a tube-shaped acoustical enclo- 
sure completely surrounded the drill 
steel. One contract (11) produced a 
form-fitting shroud-tube that closely 
surrounded the drill steel, like a 
sheath. The tube diameter was slightly 
smaller than the cutting bit diameter, 
allowing the tube to enter the hole as 



Bit-rock impact 



Piston-striking bar impact 

Leakage air noise 

Air motor noise 

Drifter-body noise 

^^ J"" Exhaust noise 




FIGURE 7. - Noise sources on jumbo-mounted drills. 



15 



drilling proceeded. The Bureau is now 
testing this "in-the-hole" shroud-tube 
design to determine its long-term dura- 
bility and acoustical effectiveness. A 
large-diameter springlike shroud tube was 
developed under the second contract and 
is now being field-tested by the con- 
tractor (14). This "spring shroud" is 
located between the drifter body and the 
front centralizer (fig. 7) and is fully 
extended before drilling begins. As the 
hole is drilled, the drifter body moves 
closer to the front centralizer, and the 
spring shroud collapses around itself. 
Unlike the form-fitting shroud tube de- 
scribed above, the spring shroud does not 
enter the drill hole; it covers only the 
portion of the drill steel that is out- 
side the hole. Noise generated by the 
drill steel that is within the hole is 
attenuated by the rock mass. 

Handheld hardrock drills are also used 
for underground ore production, espe- 
cially in tight quarters where jumbo- 
mounted drills cannot fit. Handheld 
drills are smaller and less powerful than 
jumbo-mounted drills, but the operating 
principles are the same except for the 
feed mechanism. Instead of a chain- or 
screw-type feed, handheld hardrock drills 
utilize air-powered cylinders called 
feedlegs, which are similar to the feed- 
legs used on coal mine stoper drills 
(fig. 5). Handheld hardrock drills are 
similar in design and construction to 
coal mine stopers, but must deliver more 
energy per blow to the bit because they 
drill in harder rock. The same basic 
noise-control treatments are required for 
both types of handheld drills; however, 
adapting coal mine stoper noise controls 
to hardrock drills is not a simple task 
because of the higher energy require- 
ments. A prototype quiet handheld hard- 
rock drill is now being developed under 
Bureau contract (8). 

Haulage Methods 

As in coal mines , man- or animal-pow- 
ered carts were the first step toward 
mechanization of underground ore haulage 
in hardrock mines. Although air- and 



steam-powered loading and hauling vehi- 
cles were used in the 1800' s, they were 
gradually replaced with diesel-powered 
machines. The diesel engine provides a 
safe, efficient, compact, and highly mo- 
bile power source and has been applied in 
the last 50 yr to many different types of 
underground hardrock mining equipment. 
It is by far the most popular source of 
power for today's underground hardrock 
haulage machines. 

Recent Bureau studies revealed that a 
wide variety of diesel-powered loading 
and hauling machines are now used in 
underground hardrock mines ( 26 ) . Most 
of these machines generate noise lev- 
els above 95 dBA at the operator's posi- 
tion, and operate long enough to causev 
overexposure. The noisiest machines in 
terms of operator overexposure are LHD 
machines, followed by ore trucks, ram 
haulers, tractor-trailer units, front-end 
loaders, skid-steer loaders, and shuttle 
cars. All of these machines share the 
following noise sources: (1) engine air- 
borne noise (emanating directly from the 
engine or block structure); (2) engine 
structureborne noise (engine vibration 
radiated through the structure of the ma- 
chine); (3) auxiliary-component noise — 
airborne and structureborne noise from 
the transmission, drive train, and 
hydraulic system; (4) noise generated 
by the cooling system fan; (5) exhaust 
noise — airborne noise from the outlet and 
structural radiation from the exhaust 
piping (or shell); and (6) air-intake 
noise — airborne noise from the intake 
and structural radiation from the intake 
piping. 

The Bureau directed its initial noise- 
control research efforts toward LHD vehi- 
cles because many LHD operators are over- 
exposed to noise and because the LHD is 
one of the most common machines in under- 
ground hardrock mines. Also, compared 
with other machines , the LHD is more dif- 
ficult to treat for noise control; its 
structure is more complex, it places 
greater visual demands on the operator, 
and it is not designed to accommodate an 
acoustical cab (e.g., for underground ore 



16 



trucks and surface haulage equipment) . 
It was apparent that if the noise-control 
treatments developed for LHD's were suc- 
cessful, they could be modified and ap- 
plied to other diesel-powered machines. 
As described in part 3, the Bureau is 
presently involved in both retrofit ( 22 ) 
and "factory-integration" ( 41 ) approaches 
to LHD noise control. 

Roof Support Methods 

Roof support in underground hardrock 
mining followed about the same evolution- 
ary path as in coal mining — wooden posts 
and timbers, installed manually, were 
gradually replaced by mechanized roof- 
bolting systems. Handheld and jumbo- 
mounted percussion drills are used to 
drill the holes for roof bolts; in addi- 
tion, diesel-powered machines (shotcrete 
machines, roof bolters and scalers, tran- 
sit mixers and placers, etc.) are some- 
times used in the roof support systems of 
underground hardrock mines. Because the 
diesel engine of the roof support machine 
is the primary noise-generating mecha- 
nism, the results of the Bureau's LHD 
noise-control program could be applied. 
However, the Bureau has not directly ad- 
dressed the control of noise produced by 
roof support machines (other than percus- 
sion drills) because they do not contrib- 
ute to widespread worker overexposure. 

SURFACE MINING 

Surface mining, like underground min- 
ing, involves excessive levels of equip- 
ment-generated noise. One of the first 
powered surface mining machines was a 
steam-driven "spoon dredge" developed in 
England in 1796. The need for faster 
railroad construction in the mid-1800' s 
was responsible for the development of 
today's large mining shovels. Steam pow- 
er gave way to internal-combustion and 
diesel engines, and finally to electric 
power shortly after 1900. 

Ironically, the largest and most pow- 
erful surface mining machines — shovels 
and draglines — are not responsible for 



widespread overexposure to noise because 
(1) they are comparatively few in number, 
and (2) they often have factory-installed 
noise-controlled operator cabs. Instead, 
mobile diesel-powered machines such as 
bulldozers and front-end loaders have 
been identified by the Bureau as the pri- 
mary noise offenders in surface min- 
ing operations (table 4) (9^, 40) . Even 
though these machines have about the same 
noise sources (fig. 8) as their under- 
ground counterparts, the LHD vehicles, 
the Bureau's approach to noise control 
has been much different. Since under- 
ground equipment must operate in a very 
confined environment, it is very diffi- 
cult to design and install an acoustical 
cab that does not interfere prohibitively 
with operator movement and vision. How- 
ever, this is not true of surface min- 
ing equipment , so for this equipment the 
acoustical cab approach was pursued as 
the most cost-effective means of reducing 
operator overexposure. 

TABLE 4. - "Noise offenders" in surface 
mining operations 



Equipment type 

Bulldozers 

Front-end loaders .... 

Haulage trucks 

Draglines 

Scrapers 

Overburden drills .... 

Highway trucks 

All others 



Pet of total over- 
exposed operators 

48 
15.5 

8.5 

8 

5.5 

2 
.5 
12 



Although several types and models of 
surface mining equipment now have 
factory-installed acoustical cabs, many 
older bulldozers and front-end loaders 
do not. Therefore, the Bureau developed 
acoustical cab retrofit treatments for 
two popular models of each of these two 
machines (^J) • Although each model of 
bulldozer and front-end loader requires a 
slightly different retrofit package, the 
same basic procedures can be used on all 
models. Part 3 describes the results of 
these retrofit treatments. 



17 



Cab 



A 



nz3 



I 



Exhaust 



Engine 



iZ ii 




Fan 



Main frame 



A 



Final drive 



T 
t 



A 



A 



Transmission 



KEY 
Airborne noise 



D Structureborne 
noise 



FIGURE 8. • Noise sources of mobile, diesel-powered surface mining equipment. 



PREPARATION AND PROCESSING PLANTS 

Almost all substances extracted from 
mines must undergo some form of process- 
ing to become a usable product. Mineral 
processing predates recorded history; "De 
Re Metallica," written in 1556 by Geor- 
gius Agricola, reveals the existence of 
equipment and techniques such as picking 
tables, smelting furnaces, sieves, crush- 
ers, and other items, all still used in 
one form or another. The noise associ- 
ated with coal and mineral processing 
plants became much greater as more power- 
ful energy sources, especially electric 
power, replaced wind and water as the 
primary driving forces behind mineral 
processing equipment. 

All preparation plants contain equip- 
ment that performs one of three primary 
functions: crushing (size reduction). 



screening (size separation) , and dewater- 
ing. In addition, many plants contain 
equipment that separates valuable con- 
stituents (coal or ore) from waste mate- 
rial through differences in their den- 
sities, physical properties, chemical 
properties, and/or magnetic properties. 
These machines come in a wide variety of 
shapes and sizes, and the flow of mate- 
rial through each preparation plant is 
different. Figure 9 shows the overall 
layout of the grinding mill area in a 
typical taconite (iron ore) processing 
plant. Each piece of equipment generates 
noise; furthermore, the mere transfer 
of material through the plant generates 
noise as the result of impacts between 
stationary components — chutes, bins, hop- 
pers, etc. — and the moving material. 
Fluid movement, such as air flowing 
through a size-separation or dewatering 
device, also generates noise. 



18 




FIGURE 9. •= Grinding mills in taconite processing plant. 



Noise exposures of preparation plant 
workers depend mostly on their proxi- 
mity to noise-producing equipment. Sta- 
tionary workers have specific job sta- 
tions that may or may not be close to a 
loud noise source. The noise exposures 
of stationary workers are relatively 
easy to measure compared to those of mo- 
bile workers — mechanics, samplers, clean- 
up workers, supervisors, etc. — who move 
throughout the plant during their normal 
working day. (Note the walkways in fig- 
ure 9.) Consequently, the approach for 
reducing worker overexposure may be dif- 
ferent for each type of worker. Acousti- 
cal booths and/or engineering noise con- 
trols are often better for stationary 
workers, while hearing protectors and/or 
administrative controls may be better for 
mobile workers. 



Regardless of the nature of worker 
overexposure, however, engineering noise 
control of preparation plant equipment, 
if feasible, is the best long-term solu- 
tion to the problem. The Bureau has 
therefore identified the most serious 
noise offenders in coal and mineral pro- 
cessing plants and has investigated means 
to control them. Noise generated by 
crushing devices can be controlled effec- 
tively by surrounding either the operator 
or the entire crusher with an acoustical 
barrier. Screening noise can be con- 
trolled by enclosing the area of the 
plant containing the screens or by using 
screen decks made of nonmetallic, energy- 
absorbing materials. Chutes and bins can 
be lined with rubber or other energy- 
absorbing materials to reduce the sever- 
ity of impacts caused by moving material; 



1 



19 



damping materials can also be added to 
reduce the vibration of chute and bin 
panels. An acoustical operator booth can 
be the most cost-effective solution, es- 
pecially in small, portable mineral pro- 
cessing plants. 



Part 3 describes the Bureau's prepara- 
tion plant noise-control programs in more 
detail. A related study dealing with 
the screening efficiency of nonmetallic 
screen decks is also described in part 3. 



PART 3. —RESULTS OF SELECTED RESEARCH PROGRAMS 



This part of the report describes in 
detail the results of selected Bureau 
noise-control research projects. As in 
part 2, noise-control projects for under- 
ground coal mining are discussed first, 
followed by projects involving under- 
ground hardrock mining, surface mining, 
and preparation and processing plants. 
The final section of part 3 discusses 
the results of recent Bureau research 
into the potential effectiveness of per- 
sonal hearing protectors in the mining 
environment. 

UNDERGROUND COAL MINING 

Part 2 reviewed the Bureau's under- 
ground coal mining noise-control research 
programs in terms of the three unit oper- 
ations involved — extraction, haulage, and 
roof support. Here, the results of four 
specific Bureau noise-control research 
programs are discussed in detail, again 
in terms of unit operations: (1) coal 
cutting (extraction), (2) chain convey- 
ors (material haulage), (3) mantrip vehi- 
cles (personnel haulage), and (4) stoper 
drills (roof support). Although several 
other projects dealing with underground 
coal mining noise have been conducted by 
the Bureau, the results of these four 
programs appear to have the most poten- 
tial for impact on the industry. 

Coal Cutting 

Two major projects concerned with 
coal-cutting noise have been completed 
thus far; one was a series of extensive 
laboratory studies into the mechanics 
of the coal-cutting process, and the 
other was the development of reduced- 
noise cutting heads for auger-type con- 
tinuous miners. The results of these 
programs are now being used in cur- 
rent Bureau research into the reduction 



of coal-cutting noise and will contin- 
ue to influence the direction of this 
research. 

Laboratory Studies (J_, 3) 

Early Bureau experiments revealed that 
an in-depth scientific understanding of 
the coal-cutting process was needed 
to devise an effective method for con- 
trolling coal-cutting noise. The three 
components of coal-cutting noise are 
(1) fracture noise, which is produced by 
the movement of air to fill the voids re- 
sulting from the formation of cracks in 
the coal; (2) face radiation, the vibra- 
tory response of the coal face itself; 
and (3) cutting head vibration, which re- 
sults from the transfer of cutting forces 
from the bits to the rotating head assem- 
bly. The three basic goals of the labo- 
ratory studies were to (1) identify the 
dominant component of coal-cutting noise 
(fracture, face radiation, or cutting 
head vibration); (2) determine the magni- 
tude of coal- and shale-cutting forces; 
and (3) show how the coal- and shale- 
cutting noise level can change due to ma- 
chine operating parameters such as depth 
of cut, cutting speed, bit style, etc. 

Noise of Coal Fracture and Face 
Radiation 

One of the most important tools used 
in the laboratory studies was a gravity- 
powered "linear cutting apparatus" (LCA). 
The three major components of the LCA 
(fig. 10) were a sliding carriage capable 
of holding a small sample block of coal, 
a vertical guide mast on which the sample 
carriage was mounted, and an instrumented 
cutting bit on a pedestal. To operate 
the LCA, the coal sample carriage was 
raised above the instrumented bit and re- 
leased; the falling coal sample then 



20 



•~-^,' 




m'" 




'^. 


\^ 


m^ 


1 




\ 


"%l 


-if 



■«rf^ 






1 




FIGURE 10. - Linear cutting apparatus. 



21 



struck the bit and continued downward, 
resulting in a single, linear cut on one 
face of the sample. The instrumented bit 
measured the coal-cutting forces, a coun- 
terbalance system regulated the accelera- 
tion of the coal-sample carriage (i.e., 
the "cutting speed"), and the bit-mount- 
ing pedestal could be adjusted to control 
the depth of cut. 



as independent variables (i.e., they 
were controlled by the experimenters), 
five dependent variables were measured: 
cutting force, sound pressure level, 
coal-sample surface acceleration, pro- 
duction rate, and specific energy. The 
results of the LCA tests are described 
below in terms of these dependent 
variables. 



The entire LCA was enclosed in an an- 
echoic chamber — virtually no noise was 
reflected from the walls, floors, or 
ceiling. The anechoic chamber was en- 
closed in an underground building with 
thick concrete walls to prevent outside 
noises from entering the chamber. The 
hollow interiors of the carriage-guide 
mast and the bit-mounting pedestal were 
filled with sand to reduce the LCA's 
"self-noise" during cutting. Coal-cut- 
ting noise could be measured quite accu- 
rately with this test setup because it 
was the only significant noise within the 
enclosure. 

LCA tests showed how coal fracture 
and face-radiation noise was affected 
by eight different cutting parameters: 
cutting speed, depth of cut, overburden 
pressure, cut spacing, bit type, cutting 
angle, bedding plane orientation, and 
coal type. Using these eight parameters 



Cutting Force 

One of the most important findings of 
the LCA tests was that coal resists the 
advance of a cutting bit in a manner 
analagous to the action of a spring. The 
cutting force of the bit increases until 
the tensile stress in the coal initiates 
a localized brittle fracture. This frac- 
ture propagates out from the point of 
force application for a small distance. 
The bit continues to advance through 
fractured coal, meeting relatively little 
resistance until it again contacts un- 
fractured coal, when the process is 
repeated. 

Figure 11 is a graph of coal-cutting 
force versus time; it clearly shows the 
impulsive nature of the coal-cutting pro- 
cess. Note that the initial impact force 
is no higher nor longer lasting than the 
subsequent fracture events. The peak 



,000 



£ 500 

o 

u. O 



500 



Peak force = 633 lb 



Mean cutting force = 309 I b 








25 50 75 100 125 

TIME,ms 

FIGURE llo - Coal-cutting force versus time. 



50 



175 



200 



22 




2,000 4,000 10,000 



200 400 1,000 

FREQUENCY, Hz 

FIGURE 12. - Coal-cutting force (power spectral density) versus frequency. 



I 



force initiating coal fracture and the 
number of fracture events occurring with- 
in a given length of cut were found to be 
independent of cutting speed. However, 
the type of coal, depth of cut, and bit 
configuration significantly affected the 
cutting force. 

Figure 12 is a graph of coal-cutting 
force (power spectral density^) versus 
frequency, taken from the force-time 
history of figure 11. The cutting force 

'In figure 12, power spectral density 
is expressed as the force level (in deci- 
bels) divided by the frequency bandwidth 
(in hertz) in which the force is mea- 
sured. To obtain the force level, the 
measured force is compared to a reference 
force of 1 X 10"-' lb. (The decibel is 
defined in footnote 6.) 



is relatively flat (fig. 12) until the 
"cutoff frequency," the point after which 
it declines at a rate that is inversely 
proportional to the square of the fre- 
quency. The cutoff frequency is pri- 
marily a function of the coal type and 
cutting speed — the faster the cut, the 
higher the cutoff frequency. Knowledge 
of this force-versus-f requency behavior 
played a very important role in the sub- 
sequent design of noise-control treat- 
ments for coal-cutting machinery. 

In addition, the relationship between 
cutting speed and cutoff frequency could 
be exploited in terms of reducing the 
noise of existing cutting machinery. 
Since the A-weighting scale deemphasizes 
the importance of low-frequency noise, 
slower cuts would be "less noisy" than 
faster cuts with respect to human 



23 



hearing, because the forces initiating 
the noise would be predominantly low- 
frequency forces. 

Sound Pressure Level 

With a cutting speed of 96 in/s and a 
cut depth of 1.0 in, the sound pressure 
level (SPL) ranged from 89 to 102 dB, 
depending on coal type and bit type. The 
SPL increased with increasing cutting 
speed and depth of cut, and the noise 
spectra shifted to higher frequencies as 
cutting speed increased. 

Coal Sample Surface Acceleration 

The root-mean-square (RMS) values of 
face acceleration were proportional to 
the SPL's and were affected by the in- 
dependent variables in the same manner 
as the SPL's. This proportionality sug- 
gested that sample face vibration rather 
than fracture noise was the dominant 
noise source during the LCA tests. 

Production Rate 

The average production rate was mea- 
sured by dividing the total weight of 
coal produced from a linear cut by the 
time needed to make the cut. The pro- 
duction rate was found to be directly 
proportional to the cutting speed; it in- 
creased slightly with increasing overbur- 
den pressure but was unaffected by the 
angle of bit attack. The production rate 
varied with the depth of cut according to 
the relationship 

Production rate = Depth", 

where n = an experimentally determined 
exponent. The value of n ranged from 
1.29 to 1.99, depending on the coal and 
bit type. 



was measured as the ratio of power con- 
sumed (energy per unit time) to the pro- 
duction rate (mass per unit time). The 
most important variable affecting spe- 
cific energy was the depth of cut; spe- 
cific energy decreased as the depth of 
cut increased. In terms of noise con- 
trol, this finding was important because 
it showed that for equal production 
rates , deep cuts would be quieter than 
shallow cuts. 

Coal type, overburden pressure, bed- 
ding-plane orientation, and bit type also 
affected the specific energy of the cut- 
ting process. (But somewhat surprising- 
ly, cutting speed did not.) Of these 
variables , only the bit type can be 
changed by the mine operator to achieve 
quieter, more efficient cutting. Howev- 
er, the "most efficient" cutting bit in 
terms of specific energy (as measured in 
the LCA tests) may not last as long as a 
"less efficient" bit when used on operat- 
ing continuous miners or longwall shear- 
ers. More rigorous cutting tests are 
needed to confirm the mine-worthiness of 
the bits found to be "most efficient" 
when used on the LCA. 

To summarize, the results of the LCA 
tests suggested that the noise produced 
by coal fracture and face radiation could 
be reduced by increasing the cut depth, 
decreasing the cutting speed, and choos- 
ing the most efficient cutting bit. The 
Bureau has also found that deeper, slow- 
er, more efficient coal cutting reduces 
the amount of respirable dust generated 
(31) . Therefore, if these characteris- 
tics could be incorporated into future 
designs for coal-cutting machinery, mul- 
tiple health and safety benefits would 
result. 

Noise Caused by Cutting Drum Vibration 



Specific Energy 

The specific energy of a cut is the 
single best indicator of cutting effi- 
ciency; it is defined as the cutting en- 
ergy per unit of coal extracted. In the 
LCA tests, the specific energy of a cut 



As previously stated, noise generated 
by coal cutting has three components: 
fracture noise, face-radiation noise, and 
vibration of the cutting head of the min- 
ing machine. The LCA tests only investi- 
gated the first two components; there- 
fore, further laboratory tests were 



24 



required to determine the contribution of 
cutting head vibration to coal-cutting 
noise. These tests consisted of operat- 
ing a continuous mining machine in a 
large reverberation chamber — a building 
whose predictable internal sound-reflect- 
ing characteristics made it possible to 
accurately measure the sound power gener- 
ated by noise sources within. A large 
block of synthetic coal was placed in the 
reverberation chamber to serve as a cut- 
ting medium. The synthetic coal material 
was a concrete-and-coal mixture that ac- 
curately simulated the acoustic effi- 
ciency (ability to radiate sound power 
from mechanical cutting power) of real 
coal. Figure 13 shows the continuous 
miner and synthetic coal test setup. 

In order to determine the contribution 
of cutting head vibration to the overall 
noise level, the noise generated by the 
continuous miner while idling was first 
compared to the overall noise generated 
while cutting. The conveyor of the miner 
was not operated during these tests be- 
cause the presence of conveyor noise 
would have made cutting noise more diffi- 
cult to measure. Idling noise (pumps on, 
cutting drums spinning in air) was found 
to be 81.5 dBA at the operator's posi- 
tion. As shown in table 5, the overall 
noise level was measured in two cutting 
modes — sumplng (advancing forward into 
the seam) and shearing (cutting downward 
at an 8-ln sump depth) . Noise levels in 
these two modes (94.8 and 97.0 dBA, re- 
spectively) were substantially higher 
than in the idling mode. 

The surface of the miner's cutting 
drum was then wrapped with an acoustical 

TABLE 5. - Results of continuous miner 
cutting-noise tests 

(Overall sound levels, dBA) 





Cutting mode 




Sumplng 


Shearing 
(8-in 
depth) 


Untreated drum 

Treated drum 


94.8 
91.2 


97.0 
92.6 


Noise reduction, . 


3.6 


4.4 



material (vinyl-backed foam) that greatly 
reduced its ability to radiate noise. 
The differences between the noise levels 
measured while cutting with the treated 
and untreated drums (table 5) showed that 
cutting head vibration contributed an 
additional 3.6 to 4.4 dBA to the overall 
noise level. This suggested that cutting 
head vibration was Indeed a significant 
contributor to overall cutting noise. 
Due to the logarithmic nature of the dec- 
ibel scale, a 3-dB increase in noise lev- 
el is approximately equal to a doubling 
of the sound power within the reverberant 
environment, and a 6-dB increase in noise 
level represents a fourfold increase in 
sound power. Since the noise levels of 
the untreated drum tests were 3.6 to 4.4 
dBA greater than those of the treated 
drum tests, the contribution of cutting 
head vibration to the overall noise level 
was at least as great, and often greater, 
than the sum of coal-fracture noise, 
face-radiation noise, and the idling 
noise of the miner. 

The acoustical treatment used on the 
cutting drum in these experiments may 
not have completely eliminated the noise 
generated by cutting head vibration. 
This suggests that cutting head vibration 
may be even more important than implied 
above. Unfortunately, a much sturdier 
acoustical treatment than the vinyl- 
backed foam wrapping would be required 
for underground use; but to date, no 
mlneworthy acoustical treatments for cut- 
ting drums have been developed. A 3.6- 
to 4,4-dBA noise reduction through cut- 
ting drum treatments appears to be a rea- 
sonable goal; however, if the cutting 
drums themselves were redesigned, greater 
noise reductions would be possible. 

The results of the cutting head vibra- 
tion tests suggest that coal-cutting 
noise can be reduced substantially if 
coal-cutting forces are not allowed to 
excite the cutting drum. The concept 
presently being investigated by the Bu- 
reau is the "isolated cutting drum," a 
technique that Involves the complete re- 
design of the drum and a significant 
change in the bit-mounting scheme. The 
LCA tests provided valuable information 



25 




FIGURE 13. - Continuous miner in reverberation room (top) and closeup showing cutting 
head and synthetic coal (bottom). 



26 



on how coal-cutting forces are generated, 
how large they are, and how they are 
transferred from the bit block to the 
standard cutting drum. This infomiation 
was needed to select appropriate energy- 
absorbing materials for the new "iso- 
lated" drums now being designed for both 
continuous miners ( 42 ) and longwall 
shearing machines ( 43 ) . 

Reduced-Noise-Auger-Miner 
Cutting Head (26-27) 

Auger-type continuous miners are de- 
signed to extract coal from thin seams, 
approximately 26 to 50 in in height. 
Figure 14 shows how auger miners advance 
into the coal face with a sweeping mo- 
tion. The two rotating augers at the 
front of the miner cut the coal and move 
it to the chain conveyor at the center of 
the machine. The conveyor carries the 
coal to the rear of the machine and dumps 
it onto a bridge conveyor system. The 
bridge conveyor connects with a panel 
conveyor (panline) , which removes the 
coal from the face area. 

The anchor jacks (fig. 14) of most old- 
er auger miners are emplaced manually 
by "jacksetters." Newer auger miners 
do not require jacksetters, but other 



support workers (timbermen and/or cleanup 
workers) are still needed in the imme- 
diate face area, inby the operator. Be- 
cause of their close proximity to the 
cutting heads, these workers are exposed 
to more noise than most other underground 
coal miners. Noise levels in a typical 
auger-mining section are 106 to 108 dBA 
at the jacksetter position and 102 dBA 
at the operator position. Table 2 shows 
that the allowable operating time at 
these levels is only 45 to 90 min per 
shift. The standard auger cutting heads 
are by far the dominant noise sources for 
the jacksetters, timbermen, and cleanup 
workers; whereas cutting noise and con- 
veyor noise are approximately equal at 
the operator position. 

The laboratory studies described in 
the previous section greatly aided in the 
design of a reduced-noise auger cutting 
head. Since coal-cutting forces are pre- 
dominantly low-frequency forces (fig, 
12) , the auger design was based on reduc- 
tion of its vibratory response to low- 
frequency excitation forces. The cutoff 
frequency for the standard auger cutting 
head was found to be approximately 100 to 
200 Hz while cutting the synthetic coal 
seam shown in figure 13, 




Anchor 
jack 



Miner pivots on extended right 
pivot jack as it swings to right, 
making cut /. Retracted left 
pivot jack swings forward toward 
cut 2 pivot point. 




Pivoting on extended left pivot 
jack, miner swings to left through 
cut 2. Retracted right pivot jack 
advances toward cut 3 pivot point. 




Again pivoting on extended right 
pivot jack, miner swings right, 
making cut 3. Retracted left 
pivot jack moves ahead toward 
zw\4 pivot point. 



FIGURE 14. - Cutting sequence of auger-type continuous miner. 



27 



The standard auger-miner cutting head 
(fig. 15) consists of a central cylindri- 
cal core surrounded by two helixes. The 
cutting helix contains all the cutting 
bits; the bitless, or conveying helix 
helps transport cut coal to the conveyor 
of the miner. These helixes vibrate vio- 
lently, like high-powered speakers, when 
excited by coal-cutting forces. Since 
they are the primary noise sources on the 
cutting head, the design effort was di- 
rected at reducing helix vibration. 

The mass, stiffness, and damping char- 
acteristics of the helix control its 
vibrational response to coal-cutting 
forces. The first natural vibration fre- 
quency of the standard helix was found to 
be about 200 Hz. Below this frequency, 
the helix's mass and stiffness control 
its vibrational response; above 200 Hz, 
the amount of damping at its resonant 
frequencies becomes more important. 
These structural response characteristics 
indicated that the reduced-noise auger 
design would have to (1) increase the 
first natural frequency of the helix and 
(2) increase the amount of damping at its 
resonant frequencies. 

Several methods were used to stiffen 
and damp the auger helixes; however, the 
design most suitable for in-mine use was 




found to be the sand-filled auger shown 
in figure 16. First, the conveying helix 
shown in figure 15 was removed. Exten- 
sive in-mine tests of the reduced-noise 
augers showed that the single remaining 
helix was sufficient for both cutting and 
conveying. Although this change alone 
did not drastically reduce cutting noise, 
it simplified the auger substantially and 
made the subsequent stiffening and damp- 
ing treatments easier to install. 

Next, the helix was stiffened by en- 
larging the auger's core size and adding 
a "conical helix stiffener." The larger 
core helped reduce helix vibration by 
decreasing the helix's height. The coni- 
cal helix stiffener was a second helix 
that was leaned against and welded solid- 
ly to the main helix to act as a continu- 
ous support. The addition of the stiff- 
ener resulted in the formation of three 





FIGURE 15. • Standard auger-miner cutting head. 



FIGURE 16. - Reduced-noise auger-miner cutting 
head. (Top: view from front; bottom: view from rear.) 



28 



cavities with triangular cross-sections 
(figs. 16-17). The largest cavity was on 
the nonconveying (inby) side of the he- 
lix, but because of space constraints, 
the two smaller cavities were located 
on the conveying (outby) side. In-mine 
tests showed, however, that the addition 
of the stiffener to the conveying side 
did not reduce the auger's coal-carrying 
capability. 

As shown in figure 17, the triangular- 
shaped cavities were filled with sand and 
sealed. The sand added mass, which fur- 
ther reduced the vibration of the helix 
and helped dissipate its bell-like ring- 
ing sound at its resonant frequencies. 
The added mass did not hamper the auger's 
performance because it merely replaced 
the mass lost when the conveying helix 
was removed. 

A set of reduced-noise augers was 
tested for approximately 6 months in an 



underground coal mine. Although the 
miners were initially very skeptical 
about the new auger design, they were 
completely satisfied with its performance 
by the end of the test period. More im- 
portantly, the new augers resulted in 
substantial noise reductions — 10 dBA at 
the jacksetter position (98 dBA overall 
noise) and 6 dBA at the operator position 
(96 dBA overall noise). This reduction 
was sufficient to eliminate the cutting 
head as a contributor to operator noise; 
no further reduction could be achieved 
without treating the chain conveyor 
or motors (17). The 10 dBA reduction at 
the jacksetter position resulted in a 
fourfold increase in allowable operating 
time — from 45 min to almost 3 h. 

The conical helix stiffener and sand 
are relatively simple to install; they 
could be installed by most mine shops. 
However, the larger core is rather dif- 
ficult to fabricate and does not produce 



Adapter 
(6'/2-ln-lD by 8-in-OD 



by 5-in-long tube) 



lO-ln-OD by 8-in-ID 
mechanical tubing 




LEFT END VIEW 



Note: 

All dimensions are in inches. 



L5-| 



-5-1 



3/3- in radius continues. for 180°. 
Fill with dry sand before closing. 



20 lead - 

53 

65 

PLAN VIEW 



k5- 



1.5 



0.75 




RIGHT END VIEW 



(Right hand auger shown. 
Left hand opposite.) 



typical 



.^ Fill with dry sand. 



1 



SECTION B-B' 



~2 typico/ 



SECTION /l/l' 

FIGURE 17. - Fabrication drawing of reduced-noise auger. 



29 



as much noise reduction as the stiffen- 
ing and sand-filling treatments. There- 
fore, it is recommended that mine shops 
do not attempt to enlarge the auger core. 
Detailed instructions for converting 
a standard two-helix auger into a sand- 
filled single-helix reduced-noise auger 
are now available upon request from the 
Bureau, The reduced-noise augers are al- 
so available from the auger manufacturer. 

Chain Conveyors 

Part 2 briefly described the purpose 
of a chain conveyor, its major compo- 
nents, how it generates noise, and the 
Bureau's approach to conveyor noise con- 
trol. Here, the results of Bureau re- 
search on conveyor noise (16) are dis- 
cussed more thoroughly. Figure 4 (in 
part 2) shows the location of the con- 
veyor on a continuous miner; figure 18 
shows only the conveyor and points out 
the components that produce the most 
noise. The basic noise-generating mecha- 
nisms on the conveyor are (1) scraping of 
the chain and flights against the upper 
and lower decks and (2) impacts at dis- 
continuities (gaps, misalignments, etc.), 
such as those at the idler roller, takeup 
plate, guide plate, and flex plate edge, 

Aboveground Tests 

Most of the Bureau research on conveyor 
noise control was done in an aboveground 



test facility. The key element of this 
facility was the conveyor portion of a 
continuous miner, including all the com- 
ponents pictured in figure 18, Prior to 
the installation of any noise-control 
treatments, the overall noise level at 
the operator's position (next to the bend 
of the conveyor) was 101,5 dBA. The fol- 
lowing paragraphs describe the noise- 
control treatments developed for each 
noise source in figure 18 and discuss how 
each treatment contributed to the reduc- 
tion of conveyor noise. 

Figure 19 shows the basic components 
of the noise-control treatments applied 
to the conveyor decks and sidewalls. The 
top surface of the upper deck, the bot- 
tom surface of the lower deck, and the 
outer faces of the sidewalls received a 
constrained-layer damping treatment. In 
addition, the damped deck plates were 
isolated from the conveyor chain and 
flights by resilient wear strips. Both 
the damping treatment and the we'ar strips 
contained a layer of wear-resistant 
energy-absorbing polymer material sand- 
wiched between layers of steel. The wear 
strips were also backed with rubber to 
further isolate the decks from the moving 
conveyor. The upper deck wear strips 
were originally welded to the deck in 
one piece, however, the relative motion 
between adjacent conveyor sections would 
have damaged these one-piece strips 
during normal underground operation. 



Flex plate edge 



Idler roller 




Takeup plate 



Guide plate 



Lower deck 

FIGURE 18. - Noise-producing components of o continuous miner chain conveyor. 



30 



Damping material 



Damped flex plate 




Upper deck 
resilient strip 



Damped 
upper deck 



Lower deck 
resilient strip 



Damped lower deck 

FIGURE 19. - Noise-control treatments on conveyor decks and sidewalls. 



Therefore, the upper deck wear strips 
were cut, and their leading edges were 
tapered to reduce flight-strip Impacts, 
Since this created dlscoritlnultles be- 
tween adjacent conveyor sections, it re- 
duced the quieting effect of the strips; 
however, cutting the strips in this man- 
ner Increased their durability. 

Figure 19 also shows several noise- 
control treatments designed to reduce 
noise due to Impacts at the conveyor bend 
point. First, tapered metal strips were 
welded upstream of the flex plates to 
minimize impacts between the flight tips 
and the flex plate edges. Second, the 
original round-headed carriage bolts 
holding the flex plates to the sidewalls 
were replaced by countersunk flathead 
bolts, thereby reducing Impacts between 
the flights and bolt heads. Third, the 



flex plates were damped in much the same 
way as the conveyor decks and sidewalls 
were damped. Fourth, a piece of leaded 
vinyl tape was placed over a hole in the 
sidewall near the operator's ear. Howev- 
er, the tape was only a temporary treat- 
ment; the conveyor manufacturer stated 
that the entire bend point area would 
have to be redesigned to eliminate the 
hole permanently. 

Figure 20 shows the noise-control 
treatments applied to the tail section of 
the conveyor. The vertical mismatch be- 
tween the idler roller and the rear edge 
of the upper deck was improved signifi- 
cantly by installing a larger diameter 
roller. This roller was mounted on re- 
silient support slides to isolate the 
tail section from the remaining chain- 
roller impacts. Figure 20 also shows 



31 



Modified roller 



Upper tokeup 
plate resilient pad 



Lower takeup 
plate resilient pad 





bracket support 



Resilient idler 
roller support slides 

FIGURE 20. - Noise-control treatments on conveyor idler (toil) roKer and takeup pTate. 



that the takeup plate between the tail 
roller and the rear portion of the upper 
deck was mounted on resilient support 
pads. 

Table 6 summarizes the aboveground 
noise reductions resulting from the con- 
veyor noise-control treatments. (Howev- 
er, as shown, a noise increase rather 
than a reduction resulted when the upper 
deck wear strips were cut to make them 
more durable.) Not including the block- 
ing of the lower deck hole (a temporary 
treatment), the noise-control treatments 
produced an overall noise reduction of 
10.5 dBA (from 101.5 to 91.0 dBA) at the 
operator's position. The most effective 



treatments were the constrained-layer 
damping of the decks and sidewalls, the 
resilient wear strips, and the larger 
diameter idler roller; they reduced noise 
by an estimated 5, 3, and 2 dBA, respec- 
tively. These results indicate that a 
fully treated conveyor without a lower 
deck hole would have a noise level 
12.0 dBA lower than that of the untreated 
conveyor with the lower deck hole. 

The installation of the wear strips on 
the lower deck was one of the last modi- 
fications made to the conveyor because it 
was a relatively difficult procedure. 
The entire lower deck had to be removed 
from the conveyor assembly, fitted with 



32 



TABLE 6. - Summary of aboveground chain conveyor 
noise-control tests 

(Noise level at operator's position, dBA) 

Treatment Result' 



None (untreated conveyor) 

Lower deck damped 

Upper deck damped 

Installation of upper deck wear strips.,,, 

Sldewalls damped 

Large-diameter idler roller (substitution) 

Isolation of takeup plate 

Upper deck wear strips cut 

Installation of lower deck wear strips,,.. 

Lower deck hole blocked ,,. 

'Cumulative. 



101. 


5 


101, 





97, 





96, 


5 


95. 


5 


93, 





91. 


,5 


93. 





91. 


.0 


89. 


.5 



the strips, and rewelded to the conveyor 
slightly below its original position 
to compensate for the thickness of the 
strips. The reason for the comparatively 
large noise reduction (2.0 dBA) after 
this modification was that all other ma- 
jor noise sources on the conveyor had al- 
ready been treated. With the addition of 
the lower deck wear strips, the largest 
single remaining noise source (i.e. , the 
lower deck) was quieted. 

The aboveground tests showed that noise 
generated by a chain conveyor can be 
significantly reduced without drasti- 
cally changing its original design; how- 
ever, the noise-control treatments would 
be even easier to install on newly de- 
signed conveyors. Allowable operator ex- 
posure time would increase from 1.6 h 
(untreated conveyor, 101.5 dBA) to almost 
7 h (treated conveyor, 91.0 dBA) if the 
conveyor were the only noise source. The 
potential effect of these treatments on 
operating continuous miners would be to 
make conveyor noise insignificant com- 
pared to coal-cutting noise. 

Underground Tests 

The original underground test plan 
called for noise measurements on the same 
continuous miner before and after con- 
veyor noise-control treatments were in- 
stalled. However, this plan proved to be 



unfeasible because too much equipment 
downtime would have been required to in- 
stall the noise-control treatments and 
transport the miner to and from an above- 
ground shop. (The treatments were too 
complex to install underground.) There- 
fore, noise measurements were made on 
several different machines — both treated 
and untreated, but of the same model 
whenever possible. The main disadvantage 
of this approach was that a more exten- 
sive series of measurements was required 
for a statistically proper evaluation of 
the noise-control treatments. 

The constrained-layer damping of the 
conveyor decks and sidewalls was the only 
noise-control treatment evaluated exten- 
sively in the underground tests. It was 
the first, simplest, and most effective 
treatment installed aboveground, so it 
was readily accepted for underground use 
by both the equipment manufacturer and 
the mine operators. The other noise- 
control treatments described in the 
"Aboveground Tests" section involved more 
substantial design changes, so a fully- 
treated miner conveyor was never manufac- 
tured for underground use. However, some 
useful information was obtained on the 
resilient wear strips and a previously 
neglected noise source, the guide plates 
near the bend point of the lower deck 
(fig. 18). 



33 



Table 7 shows the average underground 
noise levels produced by two groups of 
continuous miners. Six miners were in 
the treated group, and eleven were in 
the "untreated" group; all were in the 
Jeffrey^ 120 series of models (120 L, 120 
M, 120 H, 120 H2, or 122). All six of 
the treated miners had damped decks and 
sidewalls, and one of these also had wear 
strips on the upper deck (fig. 19). As 
shown in table 7, noise measurements were 
taken with the miners in five different 
operating modes and with their tail booms 
in three different positions. 

Since the data in table 7 are average 
noise measurements rather than "before 
and after" measurements of the same ma- 
chine, any conclusions from them about 
the effectiveness of the noise-control 
treatments must be made cautiously. Con- 
siderable variations in noise levels were 
found within the treated and untreated 
groups, some as large as the differ- 
ences between the two groups (average 
noise reduction) . The variations within 
groups were largely attributable to dif- 
ferences in mine conditions, equipment 
models, equipment age, and quality of 
maintenance. Nevertheless, the data col- 
lected during the underground tests re- 
vealed some general trends that led to 
these three conclusions: (1) The treated 

"Reference to specific products does 
not imply endorsement by the Bureau of 
Mines . 



machines were usually quieter than the 
untreated machines; (2) Noise-level re- 
ductions were greater for operations that 
utilize the conveyor; and (3) Miners 
whose noise-control treatments were 
installed by the manufacturer were quiet- 
er than miners whose treatments were 
installed in mining company shops. In 
addition, table 7 shows that the average 
noise reduction in the "Conveyor only (no 
coal)" operating mode was much larger 
than those in the "load only" and "cut 
and load" modes. This is because the 
presence of coal on the conveyor had a 
greater muffling effect on the untreated 
machines than on the treated machines. 
In fact, the coal on the conveyor reduced 
the average noise level more than the 
damping treatment; that is, the average 
noise level of the untreated miners in 
the "load only" mode was 1.2 dBA lower 
than that of the treated miners in the 
"conveyor only" mode (97.7 versus 98.9 
dBA). 

The durability of the constrained-layer 
damping treatment was satisfactory on all 
six treated miners. No signs of failure 
were found in either the viscoelastic 
damping material or the bond between this 
material and the steel. In one case, 
this bond proved to be stronger than the 
original bond between the upper deck and 
sidewall; the conveyor continued to oper- 
ate for 2 weeks after the deck-sidewall 
bond had failed. 



TABLE 7. - Summary of underground chain conveyor noise-control tests 
(Noise level at operator's position, dBA) 





Operating mode 




Conveyor only 
(no coal) ' 


Load only2 


Cut only 


Cut and 
load2 


Idle 


Untreated miners^ ................ 


103.8 
98.9 


97.7 
95.6 


97.2 
96.3 


100.6 
97.8 


91.0 


Treated miners^ 


91.3 


Average noise reduction 


4.9 


2.1 


.9 


2.8 


-.3 



'Average from tests using 3 conveyor tail 
from operator, angled toward operator. 
^Tail boom position: straight. 
^Average; 11 miners tested. 
^Average; 6 miners tested. 



boom positions: straight, angled away 



34 



Although the resilient wear strips 
were sufficiently durable during several 
months of aboveground tests, more prob- 
lems were expected underground because 
coal could abrade the exposed rubber 
bases of the strips. However, after a 
3-month underground trial period, damage 
had occurred only where the flights 
abruptly struck the upper deck after 
rounding the drive sprocket at the front 
of the conveyor. Upstream and downstream 
of this point, the strips showed negligi- 
ble wear. 

The underground noise levels were gen- 
erally higher when the conveyor was an- 
gled than when it was straight. This was 
caused primarily by impacts between the 
conveyor flights and the guide plates 
(fig. 18) at the conveyor bend point. 
The guide plates used in the aboveground 
tests were welded to the sidewalls and 
were relatively quiet. However, some of 
the conveyors observed underground had 
pinned-on guide plates that rattled nois- 
ily due to flight-plate impacts. In 
fact, guide plate rattle was sometimes 
the dominant noise source at the opera- 
tor's position. The obvious solution to 
this problem would be to use welded-on 
rather than pinned-on guideplates. 

In summary, the underground tests 
showed that the constrained-layer damping 
treatments were both durable and effec- 
tive in reducing noise. Continuous miner 
manufacturers could easily incorporate 
these treatments into the design of new 
conveyors. Although noise reductions 
were not as dramatic underground as they 
were in the aboveground test facility, 
this was partially due to (1) other noise 
sources (coal cutting, gathering arms, 
motors, drive train, etc.), (2) the re- 
verberant nature of the underground 
environment, and (3) the muffling ef- 
fect of the coal on the conveyor. One 
significant finding was the superi- 
ority of welded-on versus pinned-on guide 
plates as a means of noise control. 
Additional underground tests of chain 
conveyors containing all the Bureau- 
developed noise-control treatments (table 



6) are needed to further verify their 
durability and acoustical effectiveness. 

Mantrip Vehicles 

Although the mantrip vehicle (a large 
rail-mounted personnel carrier) is not 
one of the loudest noise sources in un- 
derground coal mines (figure 3, part 2) , 
almost all face workers are exposed to 
mantrip noise. Since face workers' regu- 
lar jobs often expose them to high noise 
levels , the additional contribution of 
mantrip noise could easily result in 
noncompliance with Federal noise regu- 
lations. The Bureau investigated two 
slightly different approaches to mantrip 
noise control — retrofit treatments and 
the integration of quieting techniques 
into new mantrip vehicles. The goal of 
both approaches was to reduce noise to a 
maximum of 85 dBA for all passengers. 

Retrofit Treatments ( 17 ) 

The mantrip chosen for retrofit treat- 
ments (fig. 21) had four passenger com- 
partments and a central battery and motor 
compartment. Operator controls were lo- 
cated in both of the end compartments 
to allow the driver to face the direction 
of travel. The four major elements of 
the retrofit noise-control program were 
(1) aboveground diagnostic tests, (2) in- 
stallation of noise-control treatments, 
(3) aboveground testing of the retro- 
fitted vehicle, and (4) underground 
tests. 

The three major noise sources on the 
unmodified mantrip were wheel-rail inter- 
action, motor noise, and drive train 
noise. As shown in figure 22, all three 
sources generated both airborne noise 
(direct path to passengers' ears) and 
structureborne noise (vibration of the 
frame, panels, etc.). Structureborne 
noise produced by wheel-rail interaction 
(fig. 22i4) was the largest single noise 
source, followed by airborne motor noise 
(fig. 225), structureborne motor noise, 
and the sum of airborne and structure- 
borne drive train noise (fig. 22C) . 



35 




FIGURE 21. - Mantrip vehicle used in retrofit noise-control program. 



KEY 

Airborne path 
Structureborne path 



Frame 




Bushing 



Observer 



r 



zi 



Floor 




HD) {O} 



^^ 



FIGURE 22. - Mantrip noise sources and 
transmission paths. A, Wheel-rail noise; B, 
motor noise; C, drive-train noise. 



The overall noise 
unmodified mantrip 
93.5 dBA, depending 
track curvature, and 
tion. More noise was 
travel speeds, most 
creased wheel-rail 
Noise on curves was 



levels within the 
ranged from 83.5 to 
on travel speed, 
compartment loca- 
produced at higher 
ly because of in- 
interactive forces, 
higher than noise 



on straight track because of the nature 
of wheel-rail interaction during curv- 
ing. That is, a "chattering" noise was 
produced when the wheels repeatedly 
climbed the rail and fell back downward 
while negotiating a turn. Finally, noise 
in the "passenger compartments" was 
greater than in the (end) "driver com- 
partments" because the latter were far- 
ther away from both the wheels and the 
central battery-motor compartment. (See 
figure 21.) 

Three retrofit noise-control treatments 
were installed: resilient wheels, a 
sound- absorbing motor enclosure, and 
damping of the mantrip roof and sidewall 
panels. The resilient wheels (fig. 23) 
contained rubber inserts that isolated 
the wheel-rail interface from the rest 
of the vehicle, thus decreasing struc- 
tureborne wheel-rail noise. A layer of 
sound-absorbing fiberglass was attached 
to the inner surfaces of the motor enclo- 
sure to reduce airborne motor noise, and 
a thin sheet of perforated metal covered 
the fiberglass to protect it from damage. 
The roof and sidewall damping treatment 
consisted of a layer of polymer material 
constrained by a steel sheet. 

Extensive tests of the modified mantrip 
were conducted on an aboveground track. 
Noise levels in the passenger and driver 
compartments ranged from 80 to 92 dBA, 
again depending on travel speed, track 
curvature, and compartment location. 



36 




Current 
conductor 

Wheel 
center 




Rubber 
blocks 



FIGURE 23. - Resilient wheels to isolate man- 
trip from wheel-rail noise. 

Although these noise levels were general- 
ly lower than in the unmodified mantrip, 
the noise reductions were not as great as 
expected. The major reason was that the 
new, resilient wheels had not yet been 
"broken in" to conform to the track. 
That is , the sharp edges of the new re- 
silient wheels caused them to climb far- 
ther up the rail during curving than the 
worn-down, all-steel wheels of the unmod- 
ified mantrip. 



the materials could be prefabricated 
and their dimensions could be designed 
to be compatible with the mantrip. The 
factory-designed "quiet" mantrip was sim- 
ilar to the retrofitted vehicle but dif- 
fered in four basic ways: (1) It was a 
trolley-powered model (same manufactur- 
er); (2) the roof and sidewall panels 
were prefabricated damped plates; (3) a 
redesigned suspension system (fig. 24) 
replaced the resilient wheels as the 
means for isolating the mantrip structure 
from wheel-rail interactive forces; and 
(4) the electric motor was mounted on 
vibration-isolation pads and enclosed in 
a sound-absorbing structure (fig. 25). 

These treatments were evaluated by com- 
paring the noise levels of the factory- 
quieted mantrip with those of an un- 
treated vehicle of the same model, in the 
same underground mine , and under the same 
operating conditions. Noise levels with- 
in the quieted mantrip were about 84.5 
dBA, compared to 90 to 92 dBA in the un- 
treated vehicle, recorded at an average 
speed of 10 mi/h. The same trends as 
were previously mentioned with regard to 
travel speed, track curvature, and com- 
partment location were observed. 



The retrofitted mantrip was then tested 
underground. At the end of 3 months of 
testing in two different coal mines, all 
three treatments remained basically 
intact and undamaged. Evidence of wear 
was beginning to show on the resilient 
wheels. Noise levels within the mantrip 
ranged from 83 to 86.5 dBA, measured over 
a wide variety of tram speeds and track 
curvatures. These noise levels corre- 
sponded to an average speed of 10 mi/h, 
as recorded by a machine-mounted radar 
unit. 

Factory Integration of Noise 
Controls ( 15 ) 

The major problems of the retrofit 
noise-control approach were the high 
costs of labor and materials associated 
with one-of-a-kind installation. How- 
ever, factory-integration of the treat- 
ments was considered feasible because 



The noise reduction met the goal of the 
mantrip program — a vehicle with an 85-dBA 
maximum interior noise level. Important- 
ly, the factory-quieted mantrip cost less 
than 5 pet more to build than an unqui- 
eted machine of the same model. Equip- 
ment manufacturers could easily incor- 
porate the same basic noise-control 
treatments on almost any similar mantrip 
vehicle. 

Stoper Drills 

As shown in part 2 (fig. 3), stoper op- 
erators consistently experience noise 
levels greater than 115 dBA, which is far 
out of compliance with Federal noise reg- 
ulations. Control of stoper noise was 
therefore a high-priority area of Bureau 
research. Both retrofit and redesign 
techniques were used, and stoper drills 
for both coal and hardrock use were 
addressed. 



37 



Rubber seat 



Rubber bushing 



Suspension arm 



Guide plate 




iner 



Rubber 
sleeve 



Linerite 
SECTION A-4' SECTION B-B' 




Metal 
sleeve 




Metal washer 



Rubber washer 

SECTION C-C 



Rubber sheet 



/l'< 




FIGURE 24. - Redesigned suspension system of factory-quieted mantrip. 



I- in-thick fiberglass insulation on top 
and sides of motor compartment 




Rubber mount 
FIGURE 25. - Motor enclosure of factory=quieted mantrip. 



38 



Retrofit Treatments ( 38 ) 

The most effective retrofit noise- 
control treatment for a standard stoper 
(figure 5, part 2) included a jacket-type 
muffler surrounding the drill body and 
the air-exhaust port (fig. 26). Urethane 
end caps held the jacket material (a pol- 
ymer sheet) in place. Drill-steel noise 
was partially abated by placing a 6-in- 
long collar near its bottom end (fig. 
27). The collar was made of a poured-in 



urethane 
sheath. 



layer constrained by a steel 




FIGURE 26. - Wraparound jacket-type muffler 
for stoper drill. 



In tests conducted in the Bureau's 
(Pittsburgh, PA) experimental coal mine 
(fig. 28), the jacket muffler caused 
about a 13-dBA noise reduction, and the 
drill-steel collar resulted in an addi- 
tional reduction of about 2 dBA. The 
quieted noise level, 100 dBA, would per- 
mit about 2 h of operating time per 
shift, versus no permissible time for an 
untreated stoper. The entire retrofit 
package increased the total drill weight 
(including feedleg) by about 10 pet. The 
materials used were fairly inexpensive 
(about $150 in 1980). Retrofit kits sim- 
ilar to those tested are now commercially 
available for several models of stopers. 

Retrofitted stopers (jacket muffler 
only) were tested in 15 operating under- 
ground coal mines, and muf f led-versus- 
unmuffled noise reductions of 7 to 8 dBA 
were consistently obtained. The 13-dBA 
experimental noise reduction was not 
achieved because the underground mines 
did not control the drill feed rate or 
maintain the noise-control treatments as 
diligently as the Bureau did in its ex- 
perimental mine. Nevertheless, the typi- 
cal muffled noise levels (105 to 106 dBA) 
were low enough to permit a doubling of 
the allowable operating time per shift. 
Unfortunately, drilling rates with the 
modified stoper were about 15 to 50 pet 
slower than those of unmuffled stopers, 
and freezing was a common problem with 
the wraparound mufflers. 

Redesign for Noise Control (8_, 13) 

Although the stoper retrofit noise- 
control treatments were fairly success- 
ful. Bureau studies showed that greater 
noise reductions and improved drilling 
performance could be obtained by rede- 
signing the drill to incorporate noise- 
reducing features. The redesign effort 
included three major steps: (1) redesign 
of the drill-steel rotation mechanism and 
other drill parts, (2) development of a 
more effective muffler-enclosure device, 
and (3) development of a "shroud tube" to 
attenuate drill-steel noise. 



39 





FIGURE 27. - Damping collar for stoper drill steel. 




Chuck - 
Shank- 

Piston 



Air port 

Downstroke chamber 
Valve 




Rock 
Bit 

Drill rod 



Collar 



Chuck driver nut 
Drill body 

Return-stroke chamber 

Exhaust port 
Rifle bar 
Compressed air 
Pawl, ratchet 

Compressed air 



Airleg 



FIGURE 28. - Stoper with retrofit noise-control 
treatments in operation. 



FIGURE 29. - Internal components of standard 
stoper drill with rifle-bar rotation. 

A standard stoper drill (fig. 29) pro- 
duces drill-steel rotation through a 
"rifle-bar" arrangement. The oscillat- 
ing piston supplies percussive energy to 
the steel and imparts rotation on its 
backstroke through a pawl-and-ratchet 
mechanism. The steel rotates through a 
15° to 20° arc with each stroke, result- 
ing in a rotation speed of about 150 to 
200 rev/min. 



40 



Aluminum 
outer 
cover 
Front I 

exhaust ""^ 



Isolation 
mounts 




Replaceable . , 
chuck Independent 

rotation 



Valveless 
hammer 



Flexible liner 
to prevent 

ice buildup 



FIGURE 30. - Internal components of redesigned "quiet'* stoper drill. 



The rifle-bar rotation mechanism gener- 
ated a great deal of high-frequency "rat- 
tling" noise that was beyond the ef- 
fective noise-attenuating range of the 
retrofit jacket-type muffler. Therefore, 
muffler effectiveness was increased by 
using a quieter motor-and-gear arrange- 
ment to achieve the same drill-steel ro- 
tation speed. The piston no longer ini- 
tiated rotation, so it was redesigned to 
serve as the valve controlling the flow 
of compressed air within the drill. The 
annular clearance between t"he chuck and 
shank was reduced, and the collar on the 
drill steel was eliminated to reduce mis- 
alignment and rattling impacts at the top 
of the drill body. These design changes 
improved the efficiency of the drill and 
also reduced its high-frequency noise. 
Figure 30 shows the major internal com- 
ponents of the redesigned stoper drill. 

The new drill— body design necessitated 
the development of a special muffler- 
enclosure device consisting of an alumi- 
num outer cover and rubber isolation 
mounts at each end of the drill that pre- 
vented the transfer of vibration from the 
drill cylinder to the outer aluminum 



shell (fig. 30). During drilling, the 
exhaust air from the piston chamber and 
rotation motor left the acoustical enclo- 
sure through an exhaust hole near the top 
of the drill. The muffler enclosure at- 
tenuated both drill-body and air-exhaust 
noise, and a flexible deflector plate 
near the exhaust port of the piston cham- 
ber helped reduce icing. 

Another important component of the re- 
designed stoper drill was the steel 
"shroud tube" that surrounded the drill 
steel like a sheath and acted as a bar- 
rier against the noise produced by drill- 
steel vibration. The outer diameter of 
the shroud tube was small enough to allow 
it to follow the drill bit into the hole, 
and its inner diameter was large enough 
to keep it from touching the drill steel. 
The tube was connected to the top of the 
drill body through a "shroud tube damper" 
to provide isolation from drill-body 
vibration. 

Figure 31 shows the "feedleg version" 
of the redesigned handheld drill. Six of 
these "quiet" drills were manufactured 
and tested in operating mines. All six 



41 




FIGURE 31. - Redesigned "quiet" stoper on feedleg: 



produced noise levels of 98 to 105 dBA at 
the operator's position, reflecting a 10- 
to 17-dBA reduction versus the noise 
of standard stopers. Importantly, the 
weight and penetration rates of the rede- 
signed drills were approximately equal to 
those of standard stopers. In addition, 
the three machine control levers — thrust, 
hammer, and rotation — were located to- 
gether on the drill backhead (fig. 32) , 
allowing simultaneous operator control of 
all three functions. These design and 
performance features greatly aided opera- 
tor acceptance of the redesigned drill 
and illustrated the advantages of the re- 
design approach versus the retrofit ap- 
proach to stoper noise control. 

The only disadvantage of the redesigned 
stoper (other than its initial cost) was 



that the shroud tube required removal 
and replacement during the drill-steel 
changing process. Since this proved 
to be time-consuming, operators often 
drilled without the shroud tube, partial- 
ly negating the effectiveness of the re- 
designed drill. However, noise levels 
without the shroud tube were still about 
108 dBA, which was substantially lower 
than those of standard stopers or ret- 
rofitted stopers with untreated drill 
steels. 

UNDERGROUND HARDROCK MINING 

Part 2 briefly reviewed the noise prob- 
lems associated with underground hardrock 
mines. The two major underground hard- 
rock mining noise sources the Bureau has 
investigated (other than handheld drills) 



42 



Light hammer 
only (collar) 



1/2 hammer 
1/2 rotation 



Full hammer 
full rotation 



Air to leg 
and water on 

Rotation only 
and air to leg 



Rotation rate 
control 




Leg air control 



Hammer, rotation, 
water control 

FIGURE 32. - Drilling controls of redesigned 
"quiet" stoper drill. 

are jumbo-mounted pneumatic percussion 
drills and diesel-powered LHD machines. 
These machines have been identified by 
the Bureau as the two most serious noise 
offenders in terms of both the noise lev- 
els produced and the number of workers 
overexposed. 

Jumbo-Mounted Percussion Drills 

As with mantrip vehicles and stoper 
drills, the Bureau has investigated both 
retrofit and redesign measures for reduc- 
ing jumbo drill noise. A potentially 
workable retrofit package was developed 
under Bureau contract and is now being 
tested in-house to determine its long- 
term durability. The redesigned jumbo 
drill is now being field-tested by anoth- 
er contractor. As with stoper drills, 
the two major components of the noise- 
controlled jumbo drills were (1) a muf- 
fler enclosure to attenuate air-exhaust 
and drill-body noise and (2) a shroud 
tube to attenuate drill-steel noise. 

Retrofit Treatments (11) 

Figures 33 and 34 show the retrofit 
muffler enclosure designed by the Bureau 
contractor for a drifter with rifle-bar 



rotation. This muffler enclosure had to 
surround the drifter completely because 
there were three air-exhaust ports — a 
main port on the top and two auxiliary 
ports on the underside of the drill body. 
The halves of this two-piece, boxlike en- 
closure fit together snugly around a hor- 
izontal centerline. Its octagon-shaped 
profile was a compromise reached after 
considering the requirements of exterior 
slimness and light weight and require- 
ments regarding interior volume and 
noise-attenuating properties. The top 
portion of the enclosure was hinged to 
the bottom portion to allow easy access 
to the drill (fig. 33). Figure 34 shows 
the retrofitted drill in its normal 
drilling position (cover closed). 

Figure' 35 shows the exit path of the 
main exhaust airflow (arrows). The ex- 
haust air exited the drill radially, 
struck a silicone rubber deflector (not 
shown) at the top of the enclosure, and 
moved forward to escape through an open- 
ing at the front of the enclosure. Be- 
cause the deflector was very flexible, it 
shook off any ice that began to form on 
it. The fiberglass in the muffler sec- 
tion at the front of the enclosure was 
held in place by a perforated metal 
plate, and a thin layer of Mylar polyes- 
ter film prevented it from absorbing oil 
and water. This muffler section can also 
be seen in figure 33. The three major 
advantages of this muffler-enclosure de- 
sign were that (1) exhaust noise was 
directed away from the operator and ab- 
sorbed; (2) the cold exhaust air cooled 
the coupling and shank at the front of 
the drifter; and (3) the warm drill com- 
ponents heated the exhaust air, thus in- 
hibiting ice formation. 

Figure 36 shows the components of the 
shroud tube surrounding the drill steel. 
The outer diameter of the shroud tube was 
slightly smaller than the bit diameter, 
allowing the tube to enter the hole be- 
hind the bit. The inner poljoner layer 
rode loosely on the drill steel, causing 
the tube to rotate slightly during opera- 
tion. The foam interlayer absorbed some 
of the vibration imparted to the polymer. 



43 




FIGURE 33. - Jumbo drill within retrofit muffler enclosure (cover open). 




FIGURE 34. - Jumbo drill within retrofit muffler enclosure (cover closed). 



44 



Perforated plate 
muffler section 



Enclosure 



Muffler section 

(fiberglass retained by 

perforated plate) 



Tapered exhaust exit 

transition 



Z-bar 
clamp 




"^Muffler section 



Enclosure 



'Feed channel 
FIGURE 35. - Schematic views of retrofit muffler enclosure for jumbo drill. 



Drifter 
enclosure 



Drill steel 
Plastic liner- 
Foam interlayer7 
Steel tube. 





Coupling cover 
'Coupling 

FIGURE 36. - Retrofit shroud tube for control- 
ling jumbo drill-steel noise. 

and the steel outer layer protected the 
two inner layers from dacaage. Exhaust 
air from the muffler enclosure traveled 
forward through the annulus between the 
steel and the shroud tube, escaping just 
behind the bit. 

Performance of the jumbo drill with and 
without the retrofit noise-control treat- 
ments was evaluated first in the labora- 
tory, then at an aboveground test site. 
Laboratory tests in a reverberation room 
(figures 33 and 34) showed that the sound 



power level of the treated drill was 19.3 
dBA lower than that of the untreated 
drill. At the aboveground test site, 
noise reductions of 16.5 to 18.5 dBA were 
recorded at the operator's position (ta- 
ble 8). Diagnostic tests showed that the 
muffler enclosure accounted for about 11 
dBA of this reduction; the shroud tube 
and/or the rock mass surrounding the 
drill hole accounted for the remainder. 
Ice formation and damage to the noise- 
control treatments were almost negligible 
during these tests. 

The retrofitted drill was then tested 
in an operating underground zinc mine 
(fig. 37). As shown in table 8, noise 
levels at the operator's position were 
12.5 to 15 dBA lower than those of the 
untreated drill. One of the reasons for 
the more modest noise reductions in 
the underground tests was that the con- 
fined, reverberant underground environ- 
ment resulted in reflections that par- 
tially negated the advantage of directing 
the exhaust air away from the operator. 
Also, the overall noise levels were much 
higher underground than aboveground. The 
noise levels of the treated drill indi- 
cate that it could have been operated for 
about 8 h per shift aboveground (88 to 92 
dBA) or about 1-1/2 h underground (101 to 
105 dBA). 



45 




FIGURE 37. - Jumbo drill with retrofit noise-control treatments in underground zinc mine. 



TABLE 8. - Results of retrofit jumbo drill noise-control tests 
(Noise level at operator's position, dBA) 





Position of drill' 




Collaring hole 
(10 ft steel) 


Middle of hole 
(5-6 ft steel) 


End of hole 
(1-2 ft steel) 


ABOVEGROUND TESTS 


Untreated drill 


108.5 
92 


107 
88.5 


105.5 


Treated drill 


88 


Noise reduction, . . . 


16.5 


18.5 


17.5 


UNDERGROUND TESTS 


Untreated drill 


117.5 
105 


116 
101 


115.5 


Treated drill 


101.5 


Noise reduction. , , , 


12.5 


15 


14 



'Holes were drilled to 10 ft. 

The durability of the noise-control 
treatments was evaluated by drilling ap- 
proximately 10,000 ft of hole in the un- 
derground zinc mine. Although only about 
500 ft was drilled with the shroud tube, 9 
the muffler enclosure was used during 
the entire test period. Overall, the 

^The mine did not have the 10-ft-long 
drill steels for which the shroud tube 
was designed; by the time they were ob- 
tained, the test was almost completed. 



components of the muffler enclosure were 
quite durable; the outside was not dam- 
aged, the fiberglass baffles were in good 
condition, the protective film had only 
two small holes, and the rubber exhaust 
deflector showed no signs of wear. The 
only damaged acoustical component was a 
rubber seal at the drilling-air inlet 
(fig. 35) that came off when the bolts 
supporting the drill mounting bracket 
failed. This failure, however, was not 
the fault of the acoustical treatments. 



46 



Mine personnel reported very good oper- 
ator acceptance of the partially quieted 
drill (muffler enclosure only) during un- 
derground tests, despite the need to fix 
damaged drill parts and support brackets 
on several occasions. The presence of 
the muffler enclosure did not interfere 
significantly with either the replacement 
of broken parts or routine drill mainte- 
nance. Operators generally agreed that 
the treated machine drilled just as fast 
or faster than the unmodified drills used 
at the mine. 

The Bureau is presently testing the 
fully quieted drill to evaluate the dura- 
bility of the shroud tube depicted in 
figure 36 and other similar shroud-tube 
designs. It is also evaluating the ef- 
fect of the shroud tube on operator ac- 
ceptance (e.g. , with respect to abil- 
ity to observe a stoppage of drill-steel 
rotation) . 

Redesign for Noise Control (14) 

Although the retrofit muffler-enclosure 
described above would be quite effective 
for most jumbo drills with rifle-bar ro- 
tation, it would not be appropriate for 
drifters containing independent drill- 
steel rotation motors. This is because 
independent rotation drills are usually 
somewhat larger than rifle-bar drills and 
would require larger, heavier muffler en- 
closures. The problem of air exhaust 
from the rotation motor would also have 
to be addressed. Therefore, the Bureau 
sponsored a program to redesign an inde- 
pendent-rotation drill for the purpose of 
reducing noise. 

Figure 38 is a photograph of the proto- 
type redesigned jumbo drill immediately 
prior to drilling. In order to make a 
simple, compact muffler enclosure for the 
drifter, the rotation motor was removed 
from the drifter body and relocated at 
the front end of the feed channel. This 
design change required the use of a very 
long drill shank called a kelly bar. The 
drifter percussed the rear end of the 
kelly bar while the new rotation mecha- 
nism (an air motor, belt drive, and 
gears) imparted rotation to its front 



end. The kelly-bar drive mechanism was 
then fitted with a muffler enclosure (not 
shown in figure 38) to attenuate its 
noise. 

The muffler enclosure for the drifter 
was a two-piece boxlike structure made 
entirely of molded pol5nner material. The 
top half fit snugly atop the bottom half 
and could be removed for easy access 
to the drill. The drifter was mounted 
within the bottom half of the enclosure 
through rubber bushings that isolated the 
feed channel from drifter vibration. 

The shroud tube, which is also shown in 
figure 38, was a collapsible steel coil 
approximately 8 in in diameter. Unlike 
the shroud tube on the retrofitted jumbo 
drill, it did not touch the drill steel 
or enter the hole during drilling. In- 
stead, it was suspended firmly between 
the front portion of the drifter enclo- 
sure and the rear face of the kelly- 
bar rotation mechanism. The springlike 
shroud tube was completely extended at 
the start of the drilling (fig. 38) and 
collapsed as the drifter moved toward the 
face (fig. 39). Exhaust air from the 
drifter moved forward through the shroud 
tube and a plunger-shaped rubber "sting- 
er" that was pressed against the rock 
face to attenuate noise produced by 
bit-rock interaction. Both the drifter 
exhaust air and the hole-flushing air 
exited through the small gap between the 
"stinger" and the rock face (fig. 39). 

Initial testing of the redesigned drill 
was conducted in a surface rock quarry 
(figures 38 and 39) and in a nonproduc- 
tion setting at the Colorado School of 
Mines' underground experimental mine to 
determine the acoustical performance and 
durability of the redesigned drill compo- 
nents. Noise levels at the operator's 
position were about 96.5 dBA on the sur- 
face and 100 dBA underground, reflecting 
a substantial improvement over those of 
standard jumbo drills. The redesigned 
drill has not yet been tested in an un- 
derground producing mine. At present, it 
is being modified to accommodate a semi- 
automatic drill-steel changing device 
that should facilitate long-hole drilling 



47 




FIGURE 38. - Redesigned, noise-controlled jumbo drill at start of hole. 




FIGURE 39. - Redesigned, noise-controlled jumbo drill at completion of hole. 



48 



and increase 
acceptance. 



the likelihood of operator 



Load-Haul-Dump Machines 

As stated in part 2, the Bureau desig- 
nated the diesel-powered LHD vehicle as a 
high-priority machine in terms of noise 
control because (1) almost all LHD op- 
erators are overexposed to noise; (2) the 
LHD is one of the most common machines in 
underground hardrock mines; and (3) the 
LHD is one of the most difficult diesel- 
powered machines to noise-control. As 
with other equipment types, the Bureau 
investigated both retrofit and redesign 
approaches to the LHD noise problem. 

Retrofit Treatments (22) 

Figure 40 shows the location of the 
major noise sources of a typical LHD 
vehicle with respect to its operator. 
Diagnostic tests conducted at the LHD 
manufacturer's shop produced operator 
noise levels of 99 to 101 dBA, depending 
on the vehicle operating mode. Table 9 
shows that the three major noise sources 
on the LHD, in decreasing order of impor- 
tance, were the transmission, the diesel 
engine, and the engine cooling fan. The 
engine intake and exhaust were also sig- 
nificant noise sources; however, they 
were not treated in the retrofit program 
so that efforts could be concentrated on 



the three main sources. In table 9, the 
estimated underground noise levels of the 
untreated LHD are 1 dBA tower than the 
noise levels recorded during diagnostic 
tests in the aboveground shop because the 
underground environmrnt was expected to 
be slightly less reverberant than the 
shop. The "Required reduction" column in 
table 9 shows the noise reductions needed 
to result in a combined noise level of 
90 dBA at the operator's position during 
underground operation (85 dBA from each 
source) . 

Transmission noise was dominant be- 
cause the transmission compartment was 
immediately adjacent to the operator's 
compartment (fig. 40). Airborne trans- 
mission noise was treated by sealing sev- 
eral holes between the two compartments 
and installing comp6site foam-and-plate 
linings on the top and forward sides of 
the transmission compartment (fig. 41). 
Structureborne transmission noise was 
treated by mounting the transmission on 
specially designed rubber vibration-iso- 
lation pads and installing similar mate- 
rials around the underside of the top 
cover. 

Engine airborne noise, the second 
largest source on the LHD, was treated 
primarily by building an acoustical 
enclosure around the engine (fig. 42). 
The top, left, and right sides of the 



TABLE 9. - Breakdown of noise sources on unmodified 
LHD vehicle 

(Noise level at operator's position, dBA) 



Noise source 


Recorded 
aboveground 


Underground 
(estimated) 


Required 
reduction ' 


Transmission: 

Airborne 


99 
96 

101 

96 
88 
97 
94 


98 

95 

100 

95 
87 
96 
93 


ND 


Structureborne 

Combined. 


ND 
15 


Engine: 

Airborne. 


ND 


Structureborne 

Combined 


ND 
11 


Cooling fan 


8 



ND Not determined. 

'Reduction needed to achieve 90-dBA overall level under- 
ground (85 dBA from each source) . 



49 



Water combustion 
and air intake 

tanks - 



Engine exhaust 

Cooling fan 




Engine 
Torque 
converter Hydraulic pumps 



PLAN 



Note: 

All dimensions in inches. 




ELEVATION 

FIGURE 40. - Noise sources on typical 
diesel-powered LHD vehicle. 

enclosure were lined with noise-absorbing 
polyurethane foam. Two foam-lined cut- 
outs for air exhaust were added to the 
"belly pan" beneath the engine. Exten- 
sive analysis of this enclosure configur- 
ation showed that it would not result in 
engine overheating. Structureborne en- 
gine noise was treated by welding trian- 
gular stiffening gussets to the frame 
rails on which the engine was mounted. 

The cooling fan was located at the rear 
of the engine compartment, A fold-down 
baffle that covered the cooling fan 
outlet received the same f oam-and-plate 
treatment as the transmission cover, and 
the walls surrounding the fan outlet were 
treated with polyurethane foam (fig. 43). 

Similar acoustical foam treatments were 
installed on the interior walls of the 
torque converter compartment and the com- 
partment containing the water and fuel 
tanks. (See figure 40 for the locations 
of these components.) In addition, nu- 
merous existing LHD components had to 
be relocated or modified slightly to fa- 
cilitate the installation of the noise- 
control treatments. The total cost of 





FIGURE 41o - Sound-absorbing foam lining 
within LHD transmission compartmento 

the acoustical materials was approximate- 
ly 5 to 7 pet of the selling price of a 
new LHD vehicle. 

The noise-controlled LHD was then 
tested in an underground gypsum mine; un- 
fortunately, noise levels within the 
operator's compartment could not always 
be measured directly. ^^ However, opera- 
tor noise levels for the two loudest sta- 
tionary LHD modes (high idle and torque 
converter stall) ranged from 92.5 to 93 
dBA, which was approximately 7 to 9 dBA 
quieter than in the untreated vehicle. 
In addition, the operator NEI was approx- 
imately 103 pet when measured shortly 
after the retrofitted LHD was placed into 

"■OAt the time of this study (1977), 
sound level recording equipment was too 
bulky to place in the operator's compart- 
ment during tramming without interfering 
with operator movement. 



50 







FIGURE 42. - Retrofit acoustical enclosure for LHD engine. A, Top cover; B, right side cover; C, 
left side cover; D, belly pan treatment. 



51 




FIGURE 43. 
ment lining). 



Retrofit noise-control treatments on LHD engine cooling fan (cover and fan compart- 



underground service; 1 yr later, the NEI 
was only 89 pet. Although reduced oper- 
ating time per shift may have been par- 
tially responsible for this decrease, it 
showed that the noise-control treatments 
had not deteriorated over time. Mine 
officials reported that the noise-control 
treatments did not pose serious mainte- 
nance problems or hinder machine 
performance. 

The LHD retrofit noise-control treat- 
ments were successful only because the 
individual noise sources were proper- 
ly diagnosed and the treatments were 
carefully designed, installed, and 



maintained. Each LHD vehicle is slightly 
different, so preliminary diagnostic work 
would have to be performed before similar 
noise-control treatments could be in- 
stalled on a different LHD model. Engine 
enclosures must be designed with special 
care to prevent overheating. 

The Bureau is presently sponsoring a 
program to equip four different LHD vehi- 
cles with retrofit noise controls. Pre- 
liminary results have been only moderate- 
ly successful (noise reductions of 4 to 5 
dBA) , but the program has been signifi- 
cantly affected by problems unrelated to 
the noise-control treatments (machine 



52 



breakdowns, mine closings, poor mainte- 
nance of noise-control treatments, diffi- 
culty in locating mines that will cooper- 
ate, etc.). The Bureau, in cooperation 
with MSHA, has also initiated an in-house 
program dealing with LHD retrofit noise 
controls. 

Redesign for Noise Control (40) 

Although retrofit LHD noise-control 
treatments can be successful, incorpo- 
rating these treatments in a newly de- 
signed vehicle would proably be a more 
cost-effective approach. Under Bureau 
contract, a major LHD manufacturer incor- 
porated noise-control treatments into two 
of its new machines — one with an 8-yd^ 
bucket capacity and one with a 2-yd-^ 
bucket capacity. The redesign approach 
was somewhat similar to the retrofit 
approach; preliminary diagnostic tests 
were performed on the manufacturer's 
standard-design LHD vehicles before 
treatments were designed. Since the man- 
ufacturer was not constrained by existing 
machine dimensions, better fitting acous- 
tical treatments and mechanical compo- 
nents were designed and Installed. 

Diagnostic tests revealed that the 
three major noise sources on the stan- 
dard-design LHD's were again the trans- 
mission, engine (including intake and 
exhaust) and engine cooling fan. There- 
fore, the manufacturer installed noise- 
control treatments similar to the retro- 
fit treatments described earlier. In 
addition, the manufacturer placed exhaust 
mufflers on both sides of the engine 
(fig. 44), lined the interior surfaces of 
the operator's compartment with acousti- 
cal foam (fig. 45) , and mounted the 
transmission on rubber vibration-isola- 
tion mounts (fig. 46) . These treatments 
were much easier to incorporate in the 
redesigned machines than in the retro- 
fitted machine. 

Aboveground tests of the redesigned 
LHD's revealed a 9.3-dBA noise reduction 
for the 8-yd^ LHD (from 99.0 to 89.7 dBA) 
and a 10.3-dBA reduction for the 2-yd-^ 
LHD (from 101.1 to 90.8 dBA). The S-yd^ 




FIGURE 44„ = Exhaust muffler on redesigned, 
noise-controlled LHD vehicle, 

LHD was then sent to an underground mine 
for extended testing. The results are 
shown in table 10. Although the Initial 
underground tests were encouraging (94- 
dBA operator noise level) , the noise- 
control treatments were not maintained 
properly and were gradually removed dur- 
ing the 8-month evaluation period. The 
final noise level of 102 dBA corresponded 
to that of an untreated machine, imply- 
ing that an 8-dBA noise reduction was 
achieved by the manufacturer during the 
redesign process. The 8-dBA noise in- 
crease over the evaluation period re- 
sulted in more than a threefold decrease 
in allowable operating time (1.5 to 4.7 h 
per shift); for this reason, the lack of 
proper maintenance of the noise-control 
treatments was particularly harmful. 



53 





FIGURE 45o => LHD operator-compartment noise=control treotmentSo A, Canopy; B, foot-pedal area. 



Transmission 
mount 



Fibromount 



Cinch washer 



Bolt assembly 




Main frame 



Spacers 

FIGURE 46. - Vibration-isolation mount for LHD 
transmission. 



TABLE 10. - Results of underground tests 
of redesigned, noise-controlled LHD 







Operator 


Status of treatments 


Months 


noise 




in use 


level, 
dBA 


All treatments in place 


1 


94 


Some treatments removed 


4 


97 


All treatments removed. 


8 


102 



yr. In general, mine operators were sat- 
isfied with the durability and effective- 
ness of the noise-control treatments. 



SURFACE MINING 

As explained in part 2, the Bureau's 
surface mining noise-control research 
program has concentrated on retrofit 
acoustical cab treatments for bulldozers 
and front-end loaders. Two models of 
both machine types were selected for 
these treatments, based on their overall 
popularity in the surface mining indus- 
try. The retrofitted dozers and front- 
end loaders were field tested in surface 
coal mines for a period of about 1-1/2 



Detailed fabrication manuals, contain- 
ing photographs and illustrations that 
show how the noise-control treatments 
were installed, have been prepared for 
both bulldozers and front-end loaders. 
Numerous Bureau-sponsored workshops were 
held throughout the country to provide 
equipment users with a closer look at the 
retrofit process. Most of the workshop 
attendees found them beneficial, and many 
equipment users have since applied the 
noise-control treatments to their own 
bulldozers and front-end loaders. 



54 




FIGURE 47. - Caterpillar D-9G bulldozer (ROPS-FOPS only) with retrofit noise-control treatments. 

ROPS-FOPS canopy absorption .. ,,, 

" Muffler 



Seat seals 

Hydraulic valve 
cover and tank 
seal 



Dashboard seals 
and vibration 
isolation 




Floormat and seals 



FIGURE 48. - Noise-control treatments installed on Caterpillar D-9G bulldozer (ROPS-FOPS only). 



55 



, Bulldozers (6^) 

A breakdown of the 1977 bulldozer popu- 
lation in U.S. surface mines (table 11) 
shows that the Caterpillar model D-9 was 
by far the most popular model, compris- 
ing 47 pet of the population (40) . For 
this reason, the Bureau treated two dif- 
ferent varieties of D-9 dozers, one with 
only a roll-over-protective and falling- 
object-protective structure (ROPS-FOPS) 
and one with a complete (but not acousti- 
cal) cab. To show that retrofit noise- 
control treatments could also be applied 
successfully to another manufacturer's 
bulldozer, an International Harvester 
model TD-25C (ROPS-FOPS only) was also 
treated. 

TABLE 11. - Breakdown of bulldozers 
used in U.S. surface mines, 1977, 
by model 

Pet of 
Model t otal 

47 
17 
11 
25 



Caterpillar D-9 

Caterpillar D-8 

International Harvester TD-25. . . . 
All others 



Although the design details of the 
three machines were somewhat different, 
the same four basic treatments were used: 
(1) installing a muffler on the diesel 
engine exhaust, (2) sealing numerous 
holes in the floor and dashboard of the 
operator's station, (3) adding sound- 
absorbing materials under the ROPS-FOPS 
and under the cover of the hydraulic 
tank, and (4) installing vibration-isola- 
tion materials between the engine and 
dashboard. In addition, windshields were 
installed on the two dozers that origin- 
ally contained only ROPS-FOPS. The wind- 
shields were extremely important because 
they blocked the direct path between the 
diesel engine (the largest single noise 
source on the miachines) and the dozer 
operators. Seals were also installed 
around the doors of the cab-equipped D-9 
dozer. 

Tables 12 and 13 summarize the noise 
reductions achieved through the bulldozer 
retrofit treatments, the cost of the 



acoustical materials and hardware, and 
the labor time it took to install them. 
Table 12 shows that the operator noise 
levels after treatment were 89 to 94 dBA, 
low enough to permit 6 to 8 h of daily 
operating time without violating Federal 
noise regulations; before treatment, only 
1 to 2 h of operating time was allowed. 
The effects of the individual treat- 
ments on the three machines are described 
below. 

Caterpillar D-9G With ROPS-FOPS Only 

Figure 47 is a photograph of the 
treated dozer, and figure 48 shows the 
seven major components of the retrofit 
noise-control package. Diagnostic tests 
of the untreated dozer indicated that the 
windshield would be the single most ef- 
fective noise-control treatment, followed 
by the ROPS-FOPS canopy absorption and 
the engine exhaust muffler; therefore, 
these three treatments were installed 
first. 

Figure 49 shows how the operator noise 
level decreased as each of the seven 
treatments was added. The overall noise 
reduction was 11 dBA, but the reduction 
obtained through one treatment depended 
on the presence of the previous treat- 
ments. It can be seen from figure 49 
that the windshield alone would have 



X 



T" 



Basellne (no treatment) 



Windshield 



Windshield and absorption 



Windshield, absorption, muHler 



Windshield, absorption, 
muffler, dash treatment 



Windshield, absorption, 
muffler, dash, floor seals 



Full treatment 



90 92 94 96 98 100 102 

SOUND LEVEL AT OPERATOR'S RIGHT EAR, 
AT HIGH IDLE, dBA 



104 



106 



FIGURE 49. - Step-by-step noise reduction of 
Caterpillar D-9G bulldozer (ROPS-FOPS only). 



56 



reduced the noise by about 4 dBA, 
canopy absorption alone would 
reduced the noise by about 3 dBA, 
the exhaust muffler alone would 
reduced the noise by about 1.5 dBA. 



the negligible effect on operator noise if 

have the windshield, canopy absorption, and 

and muffler had not been installed; there- 
have fore, these three treatments were by far 

The the most important components of the ret- 

a rofit package. 



remaining treatments would have had 

TABLE 12. - Summary of bulldozer retrofit noise-control treatment results 





Operator noise level. 


Noise 

reduction, 

dBA 


Cost 


Model 


dBA 


Materials, 
1978 dollars 


Labor, 




Before 


After 


h 




treatment 


treatment 








Caterpillar D-9G: 












ROPS-FOPS only 


105 


94 


11 


825 


106 


Cab (nonacoustical) 


99I-IOO2 


892-911 


9-11 


725 


88 


International Harvester TD- 












25C; ROPS-FOPS only 


102 


91 


11 


912 


80 



'Cab doors open. ^c^^ doors closed. 



TABLE 13. - Summary of treatments and costs for bulldozer noise-control treatments 



Treatment component 



Materials (approx) , 
1978 dollars 



Labor (estimated) , h 



Welder Mechanic 



CATERPILLAR D-9G WITH ROPS-FOPS ONLY 






Windshield 


275 

115 
190 
25 
80 
55 
80 
5 


29 

4 

1 


4 
2 


16 


Sound absorption materials for FOPS 
canopy 


10 


Exhaust muffler. .......................... 


2 


Dashboard seals (and vibration isolation) . 
Floormat and seals 


4 
8 


Seat seals 


8 


Hydraulic valve cover and tank seal 

Miscellaneous 


16 
2 


Total 


825 


40 


66 



CATERPILLAR D 


-9G WITH CAB 






FOPS canopy absorption. 


150 
75 
80 

105 

95 

25 

5 


4 
2 
2 
4 


2 
2 


8 


Cab wall seals 


22 


Floormat and seals 


8 


Seat and hydraulic valve seals 


12 


Sound absorption materials for cab 
surfaces 


16 


Dashboard seals (and vibration isolation) . 
Miscellaneous 


4 
2 


Subtotal 


535 
190 


16 




72 


Muffler ' 


2 


Total 


725 


16 


74 



INTEEINATIONAL HARVESTER TD 


-25C 


WITH ROPS-FOPS ONLY 




Windshield 


537 

182 

43 

150 


8 
1 

.5 


53 


FOPS canopy— absorption materials. ......... 


8 


Floormat 


2 


Seat seals and hydraulic box seal 


7.5 


Total. 


912 
400 


9.5 




70.5 


Optional: Dashboard barrier seal 


7 



'Needed only if existing muffler is ineffective. 



57 



Caterpillar D-9G With Cab 

Figure 50 shows the six major compo- 
nents of the retrofit noise-control pack- 
age applied to the cab-equipped D-9G 
dozer. This machine already had a 
relatively new muffler, so none was 
installed; however, this would ordinarily 
have been included in the package. Since 
a windshield was already a part of the 
cab, the simpler "cab wall seal" treat- 
ment replaced the windshield treatment 
required for the D-9G dozer with ROPS- 
FOPS only. The interior walls of the 
cab were treated with the same sound- 
absorbing materials as the underside of 
the canopy. As shown in table 12, the 
operator noise level in the untreated 
cab was higher when the doors were 
closed than when they were open; this was 
because the untreated doors tended to 



rattle in their sockets when they were 
closed. After door seals were added, 
however, the operator noise level was 
lower when the doors were closed. 

Figure 51 shows how the operator noise 
level decreased as the treatments were 
added (11-dBA total reduction, cab doors 
closed). As with the D-9G dozer with 
ROPS-FOPS only, the noise reduction at 
each stage of treatment depended on the 
presence of the previously installed 
treatments. 

International Harvester TD-25C 
With ROPS-FOPS Only 

Figure 52 shows the major components of 
the retrofit noise-control package for 
the TD-25C dozer. Since the manufacturer 
had already installed an exhaust muffler 



ROPS-FOPS canopy absorption 



Seat and hydraulic 
valve seals 



Dashboard seals and 
vibration isolation 



Sound absorption on 
cab interior 




Floormat and seals 



Cab wall seals 



FIGURE 50. - Noise-control treatments on cab-equipped Caterpillar D-9G bulldozer. 



58 



1 — ' — \ — ' — r 



^ r 



-I 1 1 1 r- 



Basellne (muffler only) 



Muffler and ROPS absorption 



Muffler, absorption, cab seals 



Muffler, absorption, cab seals, floor treatment 



Muffler, absorption, cab seals, floor, 
seat treatment 



Muffler, absorption, cab seals, floor, 
seat treatment, additional absorption 



Muffler, absorption, cab seals, floor, 
seat traatmsnt, additional absorption, 
dash traatmant 



L 



J. 



_L 



80 82 



84 



86 



88 



90 



92 



94 



96 



98 100 



SOUND LEVEL AT OPERATOR'S RIGHT EAR, 
AT HIGH IDLE, dBA 



FIGURE 51. - Step-by-step noise reduction of 
cab-equipped Caterpillar D-9G bulldozer. 

on this machine, none was needed; howev- 
er, a muffler would have to be installed 
if none were present. Figure 53 shows 
how the operator noise level decreased as 
the treatments were added (11-dBA total 
reduction) , and table 13 summarizes the 
material costs and labor hours associated 
with each component of the package. 

Table 13 shows that the cost of the 
windshield was the biggest difference be- 
tween the retrofit packages for the In- 
ternational TD-25C and the Caterpillar 
D-9G with ROPS-FOPS. More materials and 
labor were needed for the TD-25C wind- 
shield because it was larger and more 
difficult to fabricate, but it provided 
more noise reduction (5 dBA) than the 
D-9G windshield (4 dBA). As with the 



other two bulldozers , the noise reduc- 
tions achieved with individual treatments 
on the TD-25C depended on the presence of 
the other treatments. The windshield and 
the ROPS-FOPS canopy absorption were the 
two most important treatments; the others 
would have been ineffective without them. 
The dashboard barrier was considered to 
be an optional treatment because its cost 
was high compared to the noise reduction 
resulting from its installation. 

Front-End Loaders (7^) 

The front-end loader ranked second af- 
ter the bulldozer, as a noise offender in 
the surface raining industry (40) (table 
4) . Although about 40 pet of the loaders 
identified during a 1977 census ( 40 ) were 
equipped with factory-designed acoustical 
cabs, the remaining 60 pet required some 
type of retrofit noise-control treatment. 
The Bureau chose two of the most popular 
loader models for the retrofit program — a 
Caterpillar 988 and an International Har- 
vester H-400 B. Both machines had non- 
acoustical operator cabs, and this made 
the retrofit treatments easier to install 
than if they had been equipped only with 
ROPS-FOPS. 

The treatments themselves were simi- 
lar to those installed on the cab- 
equipped bulldozer: (1) exhaust muf- 
flers; (2) seals around openings in the 
cab walls, doors, seats, and floors; and 
(3) sound-absorbing materials on all in- 
terior cab surfaces, including the can- 
opies. Tables 14 and 15 summarize the 
noise reductions, material costs, and la- 
bor hours associated with the retrofit 
treatments. As with the cab-equipped 
bulldozer, the noise levels in the 



TABLE 14. - Summary of front-end loader retrofit noise-control 
treatment results 





Operator noise level, dBA 


Noise 

reduction, 

dBAl 


Cost 


Model 


Before 
treatment 


After 
treatment 


Materials , 
1978 dollars2 


Labor, 
h 


Caterpillar 988 

International Harvester 
H-400 B 


993-1011 

95I.3 


901-9P 
83'-873 


11 
12 


410 
580 


29 
19 



'Cab doors closed. ^Not including exhaust muffler. ^Cab doors open. 



59 



ROPS-FOPS canopy absorption 

Windshield 



Dashboard barrier 



Floormat 




Hydraulic box seals 



FIGURE 52. - Noisercontrol treatments installed on International Harvester TD-25C bulldozer. 





1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 


1 1 




Baseline (no treatment) 














Windshield 












1- 
z 

lU 


Windshield and absorption 




t- 








< 
111 

ec 


Windshield, absorption, 
floormat 














WIndthIdd, abMrption, 
lloormat, (ttt and hydraulic 

M«lt 














Full treatment 






1 1 1 1 1 


1 


1 L. 


_l 


1 1 


1 ■ . 1 . 


I.I 



85 



90 



95 



100 



105 



SOUND LEVEL AT OPERATOR'S RIGHT EAR, 
AT HIGH IDLE, dBA 



FIGURE 53. - Step-by-step noise reduction of Inter- 
national Harvester TD-25C bulldozer. 



untreated loader cabs were the same or 
greater when the doors were closed than 
when they were open, due to the doors 
rattling in their sockets. After treat- 
ment, however, the loader cabs were 
quieter with the doors closed. The costs 
of installing exhaust mufflers are not 
included in tables 14 and 15 because the 
loaders already had mufflers. If muf- 
flers had not been present, they would 
have been essential components of the 
noise-control packages. 

Figures 54 and 55 show the retrofit 
package on the Caterpillar 988 loader and 
its results, and figures 56 and 57 do the 
same for the International H-400 B. As 
with the bulldozer retrofit packages, the 
effectiveness of each successive noise- 
control treatment depended on the pres- 
ence of previous treatments. 



60 



TABLE 15. - Summary of treatments and costs for front-end loader 
noise-control packages 



Treatment component 



Materials (approx) 



Labor (mechanic, 
estimated) , h 



CATERPILLAR 


988 






Cab wall seals .................•.••••••».••... 


$58 

200 
39 
50 

63 


7 


Sound absorption materials for canopy and 
rear cab wall. ............................... 


8 


Floormat *.••.•••••••••••••••••••••••••••••••.. 


3 


Pedestal seals. ............................... 


6 


Additional sound absorption materials for cab 
interior 


5 


Total i 


410 


29 



INTERNATIONAL HARVESTER H-400 B 



Cab wall seals 


$138 

393 

49 


8 


Sound absorption materials for cab. 


8 


Floormat 


3 


Total 


580 


19 



^In 1978 dollars for Caterpillar 988; in 1979 dollars for International Harvester 
H-400 B. 



Canopy and rear cab wa! 
sound absorption 

FloornnatXc'^ 

Additional sound 
absorption on cab 
interior 



Pedestal seals 



Cab wall seals 




FIGURE 54o - Noise^control treatments installed on Caterpillar 988 front-^end loader. 



61 





1 1 1 1 1 1 > 1 1 1 1 1 1 1 i 1 ' 1 > 1 • 




Baseline (no treatment) 










Cab seals 












1- 
z 

Ul 


Seals and absorption 




K 








< 


Seals, absorption, floormat 














Seals, absorption, floormat, 
pedestal treatment 














Full treatment 






II 


L_l 


„.I..,J.. 


J—i 


1 1 1 


1 1 . 1 



80 82 84 86 88 90 92 94 96 98 100 102 

SOUND LEVEL AT OPERATOR'S RIGHT EAR, 
AT HIGH IDLE, dBA 

FIGURE 55. - Step-by-step noise reduction 
of Caterpillar 988 front-end loader. 



PREPARATION AND PROCESSING PLANTS 

Bureau studies in the 1970' s identified 
the most serious noise problems in coal 
and mineral processing plants. Noise in 
the coal industry has been regulated 
since 1969, so most of the early Bureau 
work dealt with coal preparation plants. 
Federal noise regulations were extended 
to the metallic and nonmetallic mining 
industries in 1977, so several recent Bu- 
reau programs have addressed noise prob- 
lems in these types of processing plants. 
Since the use of nonmetallic screen decks 
is one way to reduce noise in all types 
of processing plants, the Bureau spon- 
sored an in-depth study to determine the 
screening efficiencies attainable with 
nonmetallic screen decks. The following 
sections summarize the results of Bureau 
research efforts in these areas. 



Canopy absorption 



Cab wall seal 



Cab door seals 
Floormat 

Cab wall absorption 




FIGURE 56. - Noise-control treatments installed on International Harvester H-400 B front-end loader. 



62 





1 1 1 I 1 1 1 1 1 1 1 1 1 1 1 I 1 1 1 1 




t- 


Baseline (no treatment) 




z 








s 

1- 
< 


Seals and absorption 




lU 

rr 








i- 


Full treatment 






.1 


1 


1 1 1 1 1 1 1 1 1 


u 



74 



76 78 80 82 



84 



86 88 90 92 94 96 



SOUND LEVEL AT OPERATOR'S RIGHT EAR, 
AT HIGH IDLE, dBA 

FIGURE 57. - Step-by-step noise reduction of 
International Harvester H-400 B front-end loader. 



Coal Preparation Plants 

Retrofit Treatments (32-33) 

The Bureau's most successful program 
in coal preparation plant noise control 
was a retrofit project conducted at 



the Georgetown Preparation Plant, Con- 
solidation Coal Co., Cadiz, OH. The 
work consisted of three major elements: 
(1) identification of the most serious 
noise sources, (2) selection and instal- 
lation of retrofit noise-control treat- 
ments, and (3) both short- and long-term 
evaluations of the durability and acous- 
tical effectiveness of the retrofit 
treatments. 

Figure 58 shows the flow chart of the 
Georgetown preparation plant and depicts 
most of its noise-generating equipment. 
Since workers moved throughout the plant 
during the normal working shift, the 
noise sources could not be ranked in 
terms of noise level alone; the amount of 
time spent in the noisiest areas also 
had to be considered. The plant was 



-From 1.500-ton raw coal bin 



Plus 7 in 




McNally 



Crusher 



_ i^^rusiier 
nq screens ▼ ^ 



•u^ .1^ t:^ n:^^' u^ 

1 \ 1 I 1 



1 11 1 1 



Flash 

dryers ^^^ 



Loading tracks 
and loading booms 



FIGURE 58. - Flow chart of Georgetown coal preparation plant. 



63 



therefore divided into 25 different 
"noise areas," of which 13 were iden- 
tified as either "continuous-exposure 
areas" (i.e., at least one worker was 
present during the entire shift) or "fre- 
quent-exposure areas" (workers were pres- 
ent for about 50 pet of the shift) . Ta- 
ble 16 shows the noise levels in these 13 
areas and the equipment primarily respon- 
sible for the noise in each area. Mate- 
rial falling on the chutes and bins in 
these areas also contributed heavily to 
the recorded noise levels. 

Most of the noise sources in table 16 
received one or more of the following 
noise-control treatments: (1) resilient 
screen decks, (2) resilient impact pads, 
(3) chute liners, and (4) loaded-vinyl 
curtains. The resilient screen decks 



consisted of elastomeric top surfaces 
(rubber or urethane) bonded to metal bot- 
tom layers, and the impact pads and chute 
liners were also made of elastomeric ma- 
terials. These three treatments reduced 
the noise generated by material (coal or 
rock) falling or sliding on bare steel. 
The loaded-vinyl curtains were used to 
enclose several particularly noisy pieces 
of equipment. In most cases, these cur- 
tains consisted of a series of vertical 
strips of the loaded-vinyl material, held 
together along the edges with Velcro 
fasteners (fig. 59). This construction 
allowed workers to inspect the enclosed 
equipment without removing the entire 
curtain. Table 17 describes the treat- 
ments applied to each piece of equipment 
listed in table 16. 




FIGURE 59. - Curtain around screening area in Georgetown coal preparation plant. 



64 



TABLE 16. - Short term and long term effectiveness of noise-control treatments 
at Georgetown coal preparation plant, A-weighted decibels 



Equipment 



Before 


Shortly after 


3-4 yr after 


treatment 


treatment 


treatment 


111-113 


(') 


C) 


102-104 


96-97 


(2) 


95-103 


89-92 


91-92 


101 


96 


(2) 


93-100 


91-92 


93-95 


96- 97 


93 


94 


95- 97 


93 


96 


94- 97 


91-93 


93-94 


94- 97 


92 


93-94 


96 


90 


95-99 


94- 95 


92-93 


93 


94 


90-92 


93-94 


94 


91 


92-94 



Railcar shaker , 

Classifying screens , 

Centrifugal dryers , 

Vibrating screens , 

Rock crusher , 

Clean coal desanding screens, 
Secondary sizing screens...., 

Baum j igs 

Middling shaker screens , 

Coal crushers 

Flight conveyor , 

Picking table 

Primary screens 



^Not treated; shaker operation discontinued. 
Not measured; equipment not operating at full capacity, 



The noise-control treatments listed in 
table 17 had to be evaluated in terms of 
both short-term noise reduction and long- 
term durability. Short-term noise re- 
ductions were relatively easy to assess 
through bef ore-and-af ter noise measure- 
ments in the same areas. However, the 
long-texrm acoustical performance of the 
treatments was difficult to measure quan- 
titatively because of (1) normal wear of 
the acoustical materials and (2) changes 
in the plant's operating procedures dur- 
ing the course of the project. Table 17 
describes the relative durability of many 
of the treatments; but of course, the 
listed equipment "lifetimes" would have 
to be compared to those of standard (usu- 
ally steel) components to assess their 
true value. In addition, the size compo- 
sition of the material handled by the 
plant became much coarser as surface- 
mined coal replaced underground coal^^ as 
the primary plant feed. This increased 
the wear rates of the acoustical materi- 
als and increased the "inherent noisi- 
ness" of the plant (because large parti- 
cles produce larger impacts than small 
particles). Finally, the addition of a 

^ ^ Coal produced by continuous miners is 
more finely sized than that produced by 
surface coal mining equipment. 



coal froth-flotation circuit and thermal 
dryers decreased the amount of mate- 
rial handled by some of the original 
equipment . 

Table 16 shows the noise levels mea- 
sured in the critical areas of the plant 
before treatment, shortly after treat- 
ment (when all treatments were relatively 
undamaged) , and about 3 to 4 yr after 
treatment. The noise increases after 3 
to 4 yr were due to treatment wear, in- 
creased throughput size, and/or other 
factors (including material failures) de- 
scribed in table 17. Changes in plant 
operating procedures accounted for the 
lack of long-term noise reduction data 
(table 16) for some equipment types. 

The long-term physical performance of 
the noise-control treatments varied 
greatly. The loaded-vinyl curtains gen- 
erally lasted longer than the other 
treatments simply because they were not 
exposed to physical impacts; however, 
some of the elastomeric impact pads and 
chute liners were both quieter and longer 
lasting than their steel counterparts. 
The following sections summarize the 
physical performance of the four major 
types of treatments used at the George- 
town plant: 



65 



TABLE 17. - Noise-control treatments used at Georgetown coal preparation plant 



Equipment and treatment 



Comments 



Rail car shaker: No treatment, 



Classifying screens: Resilient impact pad 
beneath infeed chute; rubber-clad steel 
decks. 

Centrifugal dryers: Loaded vinyl curtain en- 
closures; Velcro fastener strips sewn on 
curtain material. 

Vibrating screens: Rubber liners in dis- 
charge chutes; ure thane-clad steel decks. 

Rock crusher: 2 rubber impact pads in infeed 
chute (2-3/16 in thick); pads had ribbed 
profile to achieve 90° impact angle. 

Clean coal desanding screens: Plastic liners 
in receiving hoppers and discharge chutes; 
loaded-vinyl curtains separated screens from 
main aisle. 

Secondary sizing screens: All-elastomer and 
rubber-clad decks; partial covers over open 
discharge chutes; loaded-vinyl curtain 
enclosures. 

Baum jigs: Ribbed rubber impact pads at in- 
feed chutes; flat urethane impact pads in 
refuse chutes; plastic and ceramic liners in 
refuse chutes; partial covers on open chute 
tops; mufflers for air blowers. 

Middling shaker screens: Loaded-vinyl cur- 
tains around screening area (both clear and 
opaque) . 

Coal crushers : Loaded-vinyl curtains between 
crushers and main aisle; Velcro fastener 
strips glued to curtain material. 

Flight conveyor: Loaded-vinyl curtains sep- 
arated conveyor from main aisle; sewn-on 
Velcro fasteners. 

Picking table: Urethane impact pad placed 
where material dropped from upper to lower 
portion of table; plastic layer downstream 
of impact pad. 

Primary screens: Elastomer-clad steel decks; 
all resilient decks (polymer topped by rub- 
ber) ; impact pads between screen sections. 



Shaker no longer in use; plant 
switched from rail-carried to 
trucked-in feed coal. Acoustic booth 
for operator may be best treatment. 

Impact pad successful for 2 yr. 
Rubber-clad screens caused blinding. 

Enclosures still in place 4-5 yr 
after initial installation. 

Rubber liners successful. Urethane- 
steel bond separated after 7 months. 

Pads completely successful , still in- 
tact 3 yr after initial installation. 

Hopper liner wore out due to impacts, 
but discharge chute liner remained 
intact. Curtains still intact 4 yr 
after installation. 

All-elastomer decks caused blinding. 
Rubber-clad decks marginally success- 
ful. Curtains very successful. 

Both types of impact pads lasted 
longer than steel. Plastic chute 
liners wore out after 2-1/2 months, 
but ceramic liners lasted 4 yr. No 
problems with mufflers. 

Clear curtains became obscured with 
dirt, but reduced noise successfully. 

Moderately successful; glued-on fast- 
eners peeled off after 6 months, 
Sewn-on fasteners recommended. 

Curtains still intact 4 yr after ini- 
tial installation. 

Impact pad lasted 2 yr before replace- 
ment. Plastic layer wore out after 
6 months. 

Elastomer-steel bond separated easily. 
All-resilient decks remained intact 
but caused blinding. Impact pads 
lasted over 3 yr before wearing out. 



66 



Resilient Screen Decks 

The tests demonstrated that elastomer- 
clad and all-elastomer screen decks could 
reduce the noise produced by impacts of 
material on the decks. However, their 
acoustical advantage over all-steel decks 
could not be measured directly because of 
the noise generated by impacts at the 
screen feed and discharge chutes, scrap- 
ing of the screen bottom decks, and vi- 
bration of the screen drive mechanisms. 

Two operational problems also occurred 
with the resilient screen decks — "blind- 
ing" and delamination. Although "blind- 
ing" (plugging of screen holes by near- 
sized pieces of material) can occur on 
any screen, depending on screen loading 
and the amount of open area initially 
present, the resilient decks seemed to be 
more susceptible to this problem than 
all-steel decks. Older, crank-arm-type 
screens (e.g., primary screens) were par- 
ticularly vulnerable. Delamination (sep- 
aration of the elastomer and steel layers 
of the elastomer-clad screens) sometimes 
occurred before the elastomer top sur- 
faces began to show significant wear due 
to material impacts. However, several 
types of elastomer-clad screens tested at 
other preparation plants (under the same 
overall program as the Georgetown tests) 
did not cause blinding, showed no signs 
of delamination, and lasted longer than 
all-steel decks. Reasons for these con- 
flicting results could not be identified 
conclusively, 

Resilent Impact Pads 

In many cases, an elastomeric impact 
pad at the inlet or discharge point of a 
belt, screen, or chute was a very cost- 
effective means of noise control. When 
designed and installed properly, the 
long service life of an elastomeric pad 
more than compensated for its high ini- 
tial cost. However, the material impact 
angle and the elastomer thickness had to 
be chosen carefully to achieve maximum 
performance. Since a 90° impact angle 
causes the minimum elastomer wear, impact 
pads placed on inclined surfaces (not 
perpendicular to material flow) were 



ribbed to create a 90° impact angle. A 
smooth impact pad was used where the ma- 
terial flow was perpendicular or nearly 
perpendicular to the pad surface. Thick- 
er impact pads were successful in some 
areas where thinner pads had failed. 

Chute Liners 



Resilient chute liners (rubber and ure- 
thane) were more durable and more ef- 
fective in reducing noise than rigid 
(plastic and ceramic) liners due to the 
resilient liners' ability to absorb mate- 
rial impact forces. Furthermore, plastic 
liners wore out quickly when exposed to 
tumbling or impacting material flows; al- 
though ceramic tiles were more durable, 
they showed evidence of cracking over 
time. Both the resilient and plastic 
chute liners lasted longer when exposed 
to smooth, sliding material flows. 

In general, the chute liners were only 
marginally effective in open chutes due 
to the noise inherent in the material 
flow. In these cases, partial chute cov- 
ers were just as effective and some- 
what cheaper than chute liners. However, 
chute covers could only be installed 
where frequent visual monitoring was not 
essential and where surges of material 
flow did not occur. 

Loaded-Vinyl Curtains 

The flexible curtains proved to be the 
longest lasting and most effective noise- 
control treatments in the plant. They 
were effective in reducing noise produced 
by large, complicated machines and could 
be easily opened or removed when equip- 
ment inspection and maintenance were 
needed. The only conditions that would 
preclude the use of these curtains in 
other plants would be (1) extremely lim- 
ited clearance around the noisy equipment 
and/or (2) the need for constant, unob- 
structed visual monitoring of the equip- 
ment from a distance. The clear loaded- 
vinyl curtains used at the Georgetown 
plant were just as effective as opaque 
curtains in reducing noise, but they soon 
became obscured with dirt. 



67 



Designing New Plants for 
Noise Control (34) 

The Bureau's Georgetown study showed 
that noise levels in coal preparation 
plants can be reduced through retrofit 
treatments; however, it was obvious that 
the noise exposures of many workers in 
these types of plants could be lowered if 
new plants were designed in accordance 
with standard engineering noise-control 
principles. For this reason, the Bureau 
prepared a manual containing noise- 
control guidelines for designers of new 
coal preparation plants. The two major 
subjects addressed in this manual were 

(1) new plant layout and design and 

(2) noise control of new plant equipment. 

In the area of new plant layout and 
design, three basic approaches were 
suggested: (1) isolation of high-noise 
areas, (2) isolation of operator loca- 
tions and walkways, and (3) choosing in- 
herently quieter preparation processes. 
The most obvious example of the first ap- 
proach is to design the plant such that 
the noisiest equipment is located far 
away from the highest concentration of 
plant workers. Limiting the number of 
openings (walkways, chutes, drains, etc.) 
between relatively noisy floors and quiet 
floors of the plant and sealing all non- 
essential openings between these floors 
are two other techniques that can be used 
to isolate high-noise areas. The second 
approach, isolation of operator loca- 
tions and walkways, is similar to the 
first except that the worker rather than 
the equipment is the primary focus. A 
good example of worker isolation would 
be a completely enclosed gallery-type 
walkway to allow visual observation of 
process equipment (fig. 60). Remote con- 
trol and computer monitoring of all plant 
processes is probably the most logical 
combined application of the first two ap- 
proaches. The third approach, choosing 
inherently quieter processes, is limited 
to situations where two types of process 
machinery perform the same basic function 
(e.g. , "louder" vacuum disc filters ver- 
sus "quieter" bowl-type centrifuges for 



fines dewatering). Choosing one large, 
easy-to-isolate process machine over sev- 
eral smaller machines of the same type 
is a combination of the first and third 
approaches. 

Noise control of new preparation plant 
equipment can often involve the use of 
the retrofit treatments installed in the 
Georgetown plant — resilient screen decks, 
impact pads , chute liners , and loaded- 
vinyl curtains. In many cases, however, 
these and other treatments are more cost- 
effective when incorporated into the 
equipment before it is installed in the 
plant. The manual described above con- 
tains detailed descriptions of 13 differ- 
ent types of coal preparation equipment, 
discusses the noise sources on each type, 
and suggests potential noise-control al- 
ternatives. Table 18 summarizes this 
information. 

Taconite Processing Plants 

The Bureau's noise-control efforts in 
metal processing plants were directed at 
taconite (iron ore) operations. Since 
taconite is harder than most other metal- 
lic ores, it generates more noise when 
crushed and requires more powerful and 
more durable processing and handling 
equipment. Noise-control treatments for 
taconite processing plants could be ap- 
plied to other processing plants with 
similar equipment. 

The approach for controlling noise in 
taconite processing plants was about the 
same as for coal processing plants — 
the most serious noise sources within a 
typical plant were identified, the noise- 
generating mechanisms of each source 
were analyzed, and potential noise-con- 
trol treatments for each source were sug- 
gested. The most important noise sources 
identified by the Bureau were (1) second- 
ary crushers, (2) grinding mills, and 
(3) f ines-dewatering screens. Several 
treatments were subsequently installed 
and evaluated, and other practical noise- 
control treatments were identified. 



68 



TABLE 18. - Noise-control alternatives for new coal preparation equipment 



Equipment type and 
typical noise sources 

Electric motors: Cooling fan, elec- 
tromagnetic forces, mechanical noise 
(bearing, bushings). 

Air compressors: Compressed air 
exhaust, internal piston or screw 
impacts. 

Enclosed gear drives: Gear meshing, 
cooling fan. 



Vacuum pumps: Pulsating air expan- 
sion, external gear reducers. 



Fans and blowers; 
drive motors. 



Air pulsations, 



Centrifugal pumps: Water pulsations, 

particle impacts within pump , drive 

motors. 
Vibrating screens: Product-to-screen 

impacts, material flow above screen, 

drive motors. 
Chutes: Product-to-chute impacts, 

material flow within chute. 



Centrifugal dryers: Material flow 
within centrifuge, motor and gear 
noise. 

Crushers: Product-to-crusher im- 
pacts, material flow within crusher, 
motor and gear noise. 

Jigs and heavy-media vessels: Jig 
blowers and exhaust, jig elevator 
discharge, material and water flow 
within vessel. 

Cyclones: Material and water flow 
within cyclone. 

Feeders and conveyors: Material 
flow and discharge, motor and gear 
noise. 



Alternatives 

Use low-horsepower, low-speed motors whenever 
possible. "Motor mute" fan mufflers, uni- 
directional cooling fans. Sound-absorbing 
motor enclosures. 

Sound absorbing enclosures (standard on newer 
models) . 

"Quieter" gear designs (helical type, tighter 
tolerances, low ratios). Cooling-fan noise 
controls; see "electric motors." Complete 
gearbox acoustical enclosures. 

Use liquid-ring pumps (instead of lobe-type 
displacement pumps). Wrap mufflers and 
silencers with pipe lagging. Low-speed 
impellers and gearbox noise controls. 

Use centrifugal (rather than positive- 
displacement) blowers. Intake and exhaust 
mufflers. Motor noise controls. 

Use V-belt (rather than gear-drive) motors. 
Avoid concentrating pumps in one location. 
Motor and gear noise controls. 

Resilient screen decks. Barrier-type enclo- 
sures around screen area (loaded-vinyl cur- 
tains). Motor and gear noise controls. 

Reduce product impact velocity (by reducing 
drop height). Resilient chute liners (in- 
ternal. External chute-damping treatments 
or barrier-type enclosures. 

Acoustic seals for centrifuge shell. Exte- 
rior lagging or damping treatments. Motor 
and gear noise controls. 

Locate crushers away from personnel. Barrier- 
type enclosures (loaded-vinyl curtains). 
Vibration-isolated crusher mounts. 

Mufflers for jig blowers and exhaust. Chute 
noise control (e,g. , impact pads). 



Cover (with inspection doors) over cyclone 

sump. 
Cover open-topped feeders and conveyors. 

Chute noise controls (impact pads). Motor 

and gear noise controls. 



69 







FIGURE 60. - Schematic of enclosed gallery-type walkway in preparation plant. 



^ 



Secondary Crusher Enclosure (23) 

Figure 61 shows that the secondary 
crusher occupied several vertical levels 
in the processing plant. For this rea- 
son, two separate enclosures were con- 
structed — one for the lower (drive) lev- 
el, and one for the upper (adjustment) 
level. The walls of both enclosures were 
made of sheet-metal panels lined with 
sound-absorbing material; the metal kept 
noise from radiating outward, and the 
sound-absorbing material prevented re- 
verberant sound buildup within the enclo- 
sure. The drive-level enclosure had 



several doors and access panels to facil- 
itate crusher maintenance. The panels 
of the adjustment-level enclosure were 
mounted on an overhead sliding track sys- 
tem because workers frequently needed ac- 
cess to the crusher at the adjustment 
level. Ceiling panels were also used on 
the adjustment-level enclosure. 

Because of the close proximity of other 
noise sources in the crusher area (terti- 
ary crushers , screens , crusher feed sys- 
tem, etc.), the total noise reduction 
provided by the secondary crusher enclo- 
sures was difficult to assess. However, 



\ 
s 



'i 



70 




FIGURE 61. ' Secondary crusher urea in taconite processing plant. 



noise levels measured at 1 ft outside the 
enclosures during normal operation were 
as much as 6 to 8.5 dBA lower than when 
the enclosures were absent. These reduc- 
tions were recorded on the side of the 
enclosure opposite the side facing the 
"open," noisy area of the plant (i.e., 
the enclosures also served as a barrier 
against other noise sources). Noise re- 
ductions on the "open" sides of the en- 
closures were 1 to 3 dBA, despite the re- 
flection of tertiary crusher and screen 
noise from the exterior enclosure walls. 
Even though the overall noise level at 1 
ft outside the enclosure was still 97 to 
101,5 dBA, the enclosures were very ef- 
fective in reducing the noise contribu- 
tion of the secondary crusher. Other 



noise sources would have to be treated to 
achieve further noise reduction in the 
secondary crusher area. 

Noise levels within the adjustment- 
level enclosure were about the same as 
those measured before it was installed 
(85 dBA while the crusher was idling, 100 
to 101 dBA while it was crushing ore). 
However, these levels would have been 
much higher if the interior enclosure 
walls had not contained sound-absorbing 
materials. The noise level within the 
enclosure would have been about 95 dBA if 
the secondary crusher were shut off com- 
pletely (with other noise sources still 
operating) , or about 5 dBA lower than the 
noise level with the enclosure absent. 



71 



The adjustment-level enclosure could thus 
protect workers from external noise dur- 
ing crusher maintenance operations. 

Grinding Mill Treatments (J_0, 29 ) 

Rod mills, ball mills, and autoge- 
nous mills are large rotating cylindrical 
structures (figure 9, part 2) used to 
grind taconite ore. As the mill rotates 
(fig. 62) , the ore particles and/or the 
metal grinding media (balls or rods) rise 
along one side of the cylinder and fall 
back to the bottom of the other side. 
Impacts between the ore, grinding media, 
and cylinder linings cause the lower por- 
tion of the falling side of the mill to 
radiate about three times as much noise 
as the top and rising sides. For this 
reason, an 8-ft-high acoustical barrier 
(fig, 62) covering only the lower, fall- 
ing side of a semiautogenous mill was de- 
signed, installed, and evaluated. 



Falling side 



Ore particles 

and 

grinding media 




Rising side 



Mill shell 



4-in-thicl< fiberglass 
supported by studs 



/4-in plywood with 
%-lb/ft^ loaded vinyl 



Outline of 
mill shell 



Existing steel grating 

FIGURE 62. - Taconite grinding mill (top) and 
noise barrier (bottom) on falling side of mill. 



At 15 ft away from the falling side of 
the mill, noise levels were about 6 dBA 
lower than before the barrier was in- 
stalled; in the "shadow zone" close to 
the barrier, noise was reduced by about 
10 dBA, Noise reductions behind the bar- 
rier at the feed end of the mill were 
only 2 to 6 dBA because of noise produced 
by the feed chute. After installation of 
the barrier, overall noise levels at most 
locations on the falling side of the 
mill were less than 95 dBA and were lower 
than noise levels on the rising side. 
However, since most of the noise on the 
rising side was actually "falling-side 
noise" reflected upward from the mill 
floor through the grating shown in fig- 
ure 62, a loaded-vinyl mat covering the 
rising-side grating would have reduced 
this noise significantly. 

Another practical grinding mill noise- 
control treatment, rubber shell liners, 
has been used successfully by several 
European and North American companies. 
Although the main reason for their use 
is for longer wear, reduced noise is an 
added benefit. Scandanavian processing 
plants have reported noise reductions of 
about 6 dBA due to rubber mill liners, 
and indirect measurements in North Ameri- 
can plants indicated a 3- to 4-dBA reduc- 
tion. Direct noise-reduction measure- 
ments could not be obtained in North 
American plants because (1) noise levels 
of standard metal-lined mills were not 
available for comparison and (2) other 
noise sources in the vicinity of rubber- 
lined mills were often dominant. 

Rubber shell liners have been used only 
in autogenous and semiautogenous mills 
because metal grinding media would abrade 
the rubber too quickly. In addition, 
they have been effective only in "wet" 
mills, where harmful temperature in- 
creases (due to warming of the liners) 
can be avoided. However, a rubber layer 
constrained between the standard metal 
liner plates and the outer shell could 
significantly reduce the noise produced 
by ball and rod mills without the adverse 
effects of abrasion and temperature in- 
creases. Rubber washers for the liner 



72 



mounting bolts on the exterior shell sur- 
face would provide additional vibration 
isolation. Although these two noise- 
control treatments have not yet been 
tested, they would be relatively easy to 
design and install and would not inter- 
fere greatly with mill maintenance. 

Fines-Dewatering Screens 

A typical f ines-dewatering screen cir- 
cuit consists of a parallel arrangement 
of boxes in which the concave screen sur- 
faces are contained. To prevent blind- 
ing of the narrow (0.004-in) screen open- 
ings, a pneumatic rapping device strikes 
the screenbox at 2- to 10-s intervals. 
Most of the screening noise is generated 
by the rapping action and the flow of 
material across the screen. Substantial 
noise reductions of 11 to 13.5 dBA were 
achieved simply by installing sliding 
covers over the screen box, shrouding the 
rapper arm, and isolating the rapper head 
from its mounting bar at the back of the 
screenbox (fig. 63). Resilient screen 
decks such as those used in coal prepara- 
tion plants would also help reduce noise 
produced by f ines-dewatering screens, 

Nonmetallic Processing Plants (30) 

The nonmetallic mining industry is 
unique in that both stationary and port- 
able processing plants are used. The 



stationary plants contain the same types 
of noise-producing equipment as coal 
and metal processing plants — crushers, 
screens, chutes, etc. Small portable 
processing plants are common in the sand, 
gravel, and crushed stone industries. 
These plants consist of a single compo- 
nent or unit operation, such as a crusher 
and screen, mounted on a chassis that can 
be moved around the mining site or towed 
on public highways. Typical operator 
noise levels in nonmetallic processing 
plants are 95 to 110 dBA, with exposure 
times as high as 6 to 7 h. 

As with the other types of processing 
plants, a three-step approach to noise 
control was taken; it included (1) noise 
source survey and characterization, 

(2) design of retrofit treatments, and 

(3) field installation and evaluation of 
treatments. After a survey of eight typ- 
ical plants showed that the crushing and 
screening process was the primary cause 
of overexposure, retrofit treatments were 
installed in three different plants. Ta- 
ble 19 describes these treatments and 
summarizes their effectiveness. 

As shown in table 19, the treatments 
were very effective in two of the three 
processing plants. In the first plant 
(which utilized a primary jaw crusher) , 
an independently mounted air-conditioned 
operator's booth replaced an ineffective 



Bolt 
Modified grommet 



Counterweight 
arm 



Modified grommet 




Washer 
Strikeplate cover 



Rubber isolator 
Washer 



FIGURE 63. - Noise-control treatments for rapper on taconite fines screen. 



73 



TABLE 19. - Noise-control treatments installed in three nonmetallic 
processing plants 



Treatment 



Result 



Cost 



Materials , 
1981 dollars 



Labor, 
h 



PRIMARY JAW CRUSHER 



Air-conditioned control booth on 
steel structure next to crusher, 
with door to catwalk to permit 
operator inspection of crusher. 



80-dBA noise level in booth 
(compared to 97 dBA on 
catwalk) . 



5,000 



40 



INCLINED SCREEN AND SECONDARY CONE CRUSHER 



Resilient screen components (feed- 
box liner, top deck, wing liners, 
discharge lip), resilient crusher 
components (feed hopper liner, 
feed plate, feed cone liners), 
and flexible curtain around 
crusher. 



3- and 7-dBA overall noise 
reduction. Treatments 
showed only minor wear 
problems over 7 months 
(198,000 tons of crushed 
stone, sand, and gravel). 




66 



HORIZONTAL SCREEN AND SECONDARY CONE CRUSHER 



Resilient screen (impact pads on 
feed chute, feedbox liner, top 
and bottom decks, wing liners, 
discharge lip) and resilient 
crusher feed hopper liner. 



No significant noise 
reduction. ^ 



17,000 



69 



'Treatment ineffectiveness was directly traceable to operator's change of equipment 
models after completion of original treatment designs. 



crusher-mounted booth that was not air- 
conditioned. The only treatments showing 
significant wear in the second plant (in- 
clined screen and secondary cone crusher) 
were the resilient crusher feed cone 
liners and feed plate; however, the lat- 
ter failure was corrected when the origi- 
nal, undersized feed plate was replaced 
with a properly sized plate. Improperly 
sized components were also responsible 
for the ineffectiveness of the noise- 
control treatments at the third plant 
(horizontal screen and secondary cone 
crusher). Since the plant operator had 
changed both the models of the equipment 
used and the nature of the crusher feed 
after the original retrofit treatments 
were designed, extensive field modifica- 
tions and remanuf acturing were necessary. 
For example, the resilient impact pad on 
the screen feedbox failed before the dis- 
charge lip could be installed. 

Despite the failure of the retrofit 
treatments at the third plant, the Bu- 
reau's program showed that retrofit 



chase price of a 
pacity of 200 to 



noise-control treatments could be applied 
successfully to portable nonmetallic 
crushing and screening plants. Treatment 
costs were only 5 to 7 pet of the pur- 
new plant (with a ca- 
300 tons/h) ; this is 
roughly equal to the annual repair and 
replacement costs of a reasonably well- 
maintained plant. Since crusher and 
screen performance was not adversely af- 
fected by the retrofit treatments, they 
appeared to be a cost-effective means of 
noise control. 

Additional Research on 
Screen-Noise Abatement 

Resilient, nonmetallic screen decks 
have been utilized extensively as a means 
to reduce noise generated by impacts of 
material (coal, rock, etc.) on the screen 
surface. Two other promising screen 
noise-abatement techniques are isolation 
of the screen drive mechanism and damping 
of the screen sidewalls. The Bureau and 
a major screen manufacturer conducted a 



74 



joint research effort to detennine the 
effects of these three treatments on 
screen performance, durability, and noise 
reduction (20) . This effort involved the 
testing of six different commercially 
available nonmetallic screen decks using 
three different products — coal, granite 
and dolomite. The results of this re- 
search corroborated the results of previ- 
ous studies. It was found that (1) coal 
screening is a quieter process than gran- 
ite or dolomite screening because coal is 
a softer material; (2) vibration isola- 
tion and sidewall damping are durable, 
cost-effective methods of controlling the 
portion of screening noise that is unre- 
lated to material impacts; and (3) resil- 
ient screen decks can be about 2 to 7 dBA 
quieter (and more durable) than all-steel 
decks, but can cause up to a 10-pct re- 
duction in screening efficiency (i.e. , 10 
pet more blinding) . 

In order to investigate the blinding 
problem more closely, a digital computer 
model was developed to evaluate screening 
efficiency under a wide variety of oper- 
ating conditions (21). A unique feature 
of this model was its use of explicit 
fundamental relationships rather than 
empirical quantities (percent oversize 
and undersize) to predict screening effi- 
ciency. The phenomena that affect the 
approach of a single particle toward the 
screen deck (particle size, hole size, 
bed depth, etc.) were investigated in or- 
der to derive a mathematical expression 
for the probability that it would pass 
through. "Probability-of -passage" equa- 
tions simulated the percentage of mate- 
rial passing through the screen on a 
given approach, and the niomber of ap- 
proaches on a given screen was used to 
determine screening efficiency. Figure 
64 is the generalized flow chart of the 
screening-simulation algorithm. 




Read in physical 
operating data 



Calculate: 
Probabilities of passage (P) 
Number of opportunities (N) 



T 



Start simulation 
(1= I) 

-l 



Calculate bed depth(BD) 
and active depth (AD) 



Feed X If X P 



Materiel removed 



Remaining material 
for next opportunity 



1=1+1 



I < N 




Overall performance 
(output) 




FIGURE 64. 
algorithm. 



Flow chart of screening simulation 



the computer-predicted values were accu- 
rate to within ±3 pet, although discrep- 
ancies were greater for screens with non- 
standard shaking strokes and frequencies. 
To allow interested parties to make quick 
estimates of screening efficiency without 
running the entire program, a handbook 
consisting of the basic program elements 
was developed. Users of this handbook 
can make quick comparisons between the 
efficiencies attainable with standard 
versus nonmetallic screen decks for the 
same type of material. 



Input to the computer program consisted 
of detailed information on the screen 
and the material being processed. Output 
consisted of the predicted screening ef- 
ficiency and the size distribution of the 
overproduct and underproduct. Laboratory 
tests with actual screens verified that 



USE OF HEARING PROTECTORS 
IN THE MINING ENVIRONMENT 

As explained in part 1 , the Bureau 
views hearing protectors (earplugs, ear- 
muffs, etc.) as only a partial, tempo- 
rary solution to mining noise problems. 



75 



Historically, it has been more effective 
to control industrial health problems at 
their sources than to require workers to 
use personal protective devices. For ex- 
ample, early attempts to reduce coal 
miners' exposure to dust through the use 
of respirators did not prevent coal work- 
ers' pneumoconiosis. Although large dust 
particles were trapped by the respirator, 
respirable dust particles (those smaller 
than 5 ym) passed through and around the 
respirator and were inhaled by workers. 
The same is true of hearing protective 
devices; unless they achieve a perfect 
seal around the ear (earmuffs) or ear 
canal (earplugs), noise will pass through 
the openings and render them ineffective. 

However, Federal regulations do provide 
for the use of hearing protectors by 
requiring mine operators to make them 
available to miners in situations where 
engineering or administrative noise con- 
trols are not available, have not yet 
been developed, or fail to reduce noise 
to within levels of compliance. There- 
fore, the Bureau has conducted research 
to investigate the noise-reducing charac- 
teristics of personal hearing protectors 
and assess their potential usefulness in 
the mining environment. 



Limitations of Hearing 
Protector Effectiveness 

No hearing protective device can elimi- 
nate all sound transmission to the ear. 
Figure 65 shows the four most common 
noise pathways: (1) air leaks around the 
protector, (2) transmission of external 
sound through the protector material, 
(3) vibration of the protector in re- 
sponse to external noise, and (4) bone 
and tissue conduction — sound transmitted 
directly through the skull. Even if a 
"perfect" hearing protector could be de- 
veloped to eliminate the first three 
pathways, bone and tissue conduction 
would still be present. Although the 
amount of bone and tissue conduction 
varies among individuals and different 
sound frequencies, the maximum noise re- 
duction attainable with a "perfect" hear- 
ing protector would be about 50 dB (18) . 
Even under well-controlled laboratory 
conditions, the maximum noise reduc- 
tion achieved through the use of present- 
ly available hearing protectors has 
been about 35 dB. Furthermore, these 
maximum reductions would occur only in 
the frequency range of 2,000 to 4,000 
Hz, In real-world situations (with non- 
perfect protector fit, etc.), actual 





Bone 
and tissue 












































Protector 
vibration 




' 


r 




' 


' 




' 


' 


Noise 




— ^ 


Outer 
ear 




Middle 
ear 




Inner 
ear 






Material 
leaks 


— »- 








1 


1 




















Air leaks 















FIGURE 65. - Noise pathways to ear protected by hearing protective device. 



76 



noise reductions were found to be 20 dB 
or less ( 4_) , again depending on the fre- 
quency examined. 

Assessing Earmuff Attenuation 

Two basic techniques have been wide- 
ly accepted for measuring the noise- 
reducing properties of hearing protective 
devices. The "real-ear" method (American 
National Standards Institute (ANSI) stan- 
dard S3. 19-1974) measures the hearing 
threshold of a human subject both with 
and without hearing protection. The dif- 
ference in the hearing threshold of the 
protected and unprotected subject indi- 
cates the effectiveness of the hearing 
protector. Although the real-ear and 
other "psychophysical" methods of this 
type are used most often, several nonsub- 
j active, "physical" methods are also 
used. Physical methods often involve two 
microphones, one outside the hearing pro- 
tector and one between the protector and 
the subject's eardrum. ^2 The difference 
in sound level at the two microphone lo- 
cations is then measured. In both the 
psychophysical and physical approaches, 
the noise-reducing properties of the 
hearing protectors are measured over the 
entire audible frequency range. Af- 
ter applying corrections for A-weighting 
and the frequency spectrum of the inci- 
dent noise, a noise-reduction rating 
(NRR) is assigned to the hearing protec- 
tive device. 

The psychophysical (real-ear) method 
is undoubtedly more accurate than the 
physical (microphone) method of measuring 
hearing protector effectiveness. Howev- 
er, the psychophysical method requires 
the use of a laboratory and is very time 
consuming, so the physical method is pre- 
ferable for field evaluation of hearing 
protectors. In order to determine the 
exact differences between the two meth- 
ods, the Bureau conducted a series of 
controlled laboratory experiments on ear- 
muffs commonly worn by miners (36). 

"■^ANSI standard S3. 19-1 974 specifies a 
test procedure in which a dummy head with 
a simulated human skin is the "subject." 



First, real-ear attenuations were mea- 
sured for 5 different types of miners' 
earmuff s, worn by 12 different subjects 
with normal hearing "Unprotected" mea- 
surements (without muffs) were also re- 
corded. Microphones were then placed in- 
side and outside the muffs, and the same 
12 subjects were tested. 

At sound frequencies below 2,000 Hz, 
the measured earmuff attenuations were 
approximately the same in both the physi- 
cal (microphones) and psychophysical 
(real-ear) tests. Above 2,000 Hz, how- 
ever, greater attenuations were measured 
in the psychophysical tests than in the 
physical tests (3- to 7-dB difference). 
These results were consistent among all 
12 subjects and for all 5 types of ear- 
muffs. The differences probably resulted 
from the inherent differences in the two 
test procedures. In the "unprotected" 
portion of the psychophysical tests, the 
high-frequency external sounds caused a 
standing-wave resonance phenomenon to oc- 
cur within the subjects' ear canals, thus 
increasing the per-ceived sound levels. ^^ 
In the "protected" portion of the psycho- 
physical tests, both the exterior noise 
and the standing-wave phenomenon in the 
ear canal were drastically reduced. In 
contrast, the standing-wave phenomenon 
did not occur in the "unprotected" por- 
tion of the physical test because the un- 
protected microphone was completely out- 
side the muff. The attenuation measured 
at the protected microphone, therefore, 
included only the reduction of exterior 
noise. 

The existence of the standing-wave res- 
onance phenomenon in the ear canal points 
out an important limitation of the physi- 
cal method of measuring earmuff effec- 
tiveness; in fact, this limitation is 
common to almost all conventional methods 
of sound measurement: Since the pro- 
tected microphones could not be placed 

^ -^The standing-wave resonance phenome- 
non was prominent only at high frequen- 
cies because these sounds had wavelengths 
that were similar to the lengths of the 
subjects' ear canals. 



77 



Inside the subjects' ear canals, they 
did not record the sound produced by 
the standing waves. Therefore, the two- 
microphone earmuff system actually under- 
estimated the earrauffs' ability to atten- 
uate high-frequency noise. 

In order to compensate for the differ- 
ences between the psychophysical and 
physical methods of measuring earmuff 
attenuation, the Bureau designed a cor- 
rection filter for the protected micro- 
phone and used a microphone with linear 
response outside the muff. When tested 
in the laboratory, this system produced 
results that were similar to the results 
of the psychophysical tests (37). How- 
ever, the system would not reliably mea- 
sure actual earmuff attenuation in the 
field because the protected microphone 
and its cable would undoubtedly move 
around and contact the subject's body, 
thereby producing a misleadingly high 
sound level under the muff. Exterior 
sound levels would have to be at least 
100 dB at all audible frequencies to 
override this under-the-muf f noise. In 
most field situations, exterior sound 
levels would not exceed 100 dB at higher 
frequencies; therefore, applications of 
the "corrected" physical technique would 
be extremely limited. 

The Bureau is still engaged in research 
to devise a reliable physical method for 
measuring the effectiveness of miners' 
hearing protectors as worn in the field. 
Even if the two-microphone muff system 
described above could be perfected, the 
same technique would be extremely diffi- 
cult to apply to earplugs. The Bureau 
plans to attempt to improve the earmuff 
system and devise a physical measuring 
system applicable to earplugs and other 
types of hearing protectors. Until such 
systems are developed, the true benefits 
of hearing protectors in the mining envi- 
ronment cannot be determined. 



Hearing Protector Interference 
With Required Acoustical Cues 

One of the miners' greatest concerns 
about hearing protectors is the potential 
elimination of sounds they need to hear, 
such as verbal communications and "roof 
talk" — the noise preceding an impending 
roof fall. For this reason, the Bureau 
conducted research to determine the 
amount of interference caused by hearing 
protectors. One study (35) showed that 
when the ambient noise level in the mine 
was less than 90 dBA, hearing protectors 
did indeed inhibit the miners' ability to 
discriminate speech and roof talk from 
other noises. However, when the ambient 
noise level in the mine was greater than 
90 dBA, discrimination of speech and roof 
talk was the same or slightly better when 
hearing protectors were used. The basic 
conclusion of this study was that hearing 
protectors would benefit miners only when 
overall noise levels exceed 90 dBA. 

In an effort to provide a hearing pro- 
tector that would work only when noise 
exceeded 90 dBA, the Bureau developed a 
"discriminating earmuff" (12). An elec- 
trical system incorporated into the muff 
allowed all sounds lower than 83 dBA to 
pass unattenuated to the ear. Sounds 
louder than 83 dBA were progressively at- 
tenuated up to a maximum attenuation of 
30 dBA (90 dBA under the muff at 120 dBA 
exterior noise). In underground tests, 
subjects wearing the discriminating ear- 
muff were able to hear low-level sounds 
such as speech and roof talk more easily 
than subjects wearing standard muffs. 
The discriminating muffs and standard 
muffs provided equal attenuation in high- 
noise environments. The only disadvan- 
tage of the discriminating muffs was that 
their electronics made them rather expen- 
sive and too fragile for in-mine use. 



PART 4. —FUTURE BUREAU NOISE-CONTROL EFFORTS 



Parts 2 and 3 documented the most sig- 
nificant and beneficial results of past 
Bureau noise-control research programs 
and illustrated their broad scope. Most 



of these programs were directed toward 
solving the immediate noise problems of 
the mining industry and involved field 
demonstrations of retrofit techniques 



78 



whenever possible. When redesign of 
equipment appeared to be a more viable 
long-term solution, the end product 
sought by the Bureau was usually a field 
demonstration of a prototype "quiet" ma- 
chine or process. 

Recently, however, the scope and nature 
of Bureau research in all areas, includ- 
ing noise control, has changed dramati- 
cally. Demonstration projects such as 
those described in part 3 have been de- 
emphasized in favor of long-term in-house 
programs in basic noise studies and ap- 
plied noise-control research. The rea- 
sons for this change in emphasis were 
both philosophical and monetary. Limited 
resources have made it necessary for the 
Bureau to adopt the point of view that 
since a successful field demonstration of 
an inherently quieter piece of equipment 
requires substantial assistance from its 
manufacturer and user, these parties, 
rather than the Government , should take 
the leading role. Furthermore, inherent 
design differences between different 
models of the same type of mining equip- 
ment make it nearly impossible for the 
Bureau to design a "generic" noise-con- 
trol treatment suitable for any single 
equipment type. Although past Bureau 
noise-control projects have successfully 
addressed specific models, the cost of 
developing numerous model-specific treat- 
ments is prohibitive. Unfortunately, 
this is exactly the type of effort needed 
to achieve widespread, immediate solu- 
tions to the many noise problems in min- 
ing, but neither the Bureau nor private 
industry can afford this effort at the 
present time. Therefore, the Bureau is 
now pursuing long-term strategies that 
will use its limited resources more 
effectively. 

Until recently, the desire to achieve 
immediate noise reductions in many mining 
areas forced the Bureau to rely heavily 
on contracting firms, mostly noise spe- 
cialists and equipment manufacturers. 
This approach helped introduce noise- 
control technology to the mining industry 
but did not allow Bureau personnel to 
pursue basic, long-term goals in noise 



control. However, the Bureau has now 
begun to concentrate on acquiring the 
capability to perform most of its noise- 
control research in-house. Research ef- 
forts will consist of long-term studies 
of mining noise problems that cannot be 
solved by the industry itself and will 
focus only on those problems that require 
in-depth scientific study. 

FACILITIES AND EQUIPMENT 

High-quality facilities and equipment 
are essential in any noise-research pro- 
gram, and the Bureau is now establishing 
these at its Pittsburgh Research Center. 
The centerpiece of the in-house facility 
is a large reverberation building^ ^ capa- 
ble of housing a piece of mining equip- 
ment as large as a continuous miner, 
jumbo drill rig, or bulldozer. The phys- 
ical dimensions and interior surfaces of 
this building were designed specifical- 
ly to allow precise measurements of the 
sound power levels of noise sources lo- 
cated within. A small anechoic chamber, 
a room whose walls, floors, and ceiling 
reflect almost no noise, has been built 
outside the larger reverberation room 
for experiments with personal hearing 
protectors. All laboratory instrumenta- 
tion needed to conduct detailed acousti- 
cal experiments in these rooms (sound 
level meters, tape recorders, micro- 
phones , accelerometers , frequency ana- 
lyzers, etc.) have been obtained. 

In order to allow mining machines to 
operate "normally" (i.e., with all noise- 
generating mechanisms present) within the 
reverberation building, it will contain 
several different cutting media. Noise 
produced by coal-cutting machines will 
be investigated by cutting into a synthe- 
tic coal seam similar to the one shown 
in figure 13 (part 3). Noise produced 
by rock drills will be investigated 
by drilling into replaceable blocks of 
extra-hard concrete. Specific tests to 
be conducted in the reverberation build- 
ing are summarized below. 

^ '^Scheduled for completion by the end 
of 1984. 



79 



RESEARCH PROGRAMS 

Because of the relatively recent change 
in emphasis from contract to in-house re- 
search, the programs described below are 
still in their early stages. The four 
major areas of emphasis — coal cutting, 
conveying, percussion drilling, and hear- 
ing protectors — were chosen because con- 
tract research efforts have shown that 
long-term studies are still needed in 
these areas. 

Coal Cutting 

Although machine components for quieter 
coal cutting and conveying were designed 
and developed under Bureau contracts 
(part 3) , only the sand-filled auger- 
miner cutting head received extensive 
testing under operating conditions in 
underground coal mines. Quieter cutting 
heads for drum-type continuous miners and 
longwall shearers are now being tested 
underground under two remaining Bureau 
contracts. Future work in this area will 
include basic studies into the effects 
of bit lacing and geometry on noise 
and the investigation of alternative, 
quieter cutting technologies (e.g. , water 
jetting). 

Conveying 

The "quiet" chain conveyor components 
that performed well in aboveground tests 
could not be tested extensively under- 
ground due to problems in locating coop- 
erating mines, manufacturers' reluctance 
to incorporate the unproven components in 
their product lines, and the short-term 
nature of the contract under which the 
components were developed. The long-term 
durability and acoustical performance of 
these "quiet" components are now being 
tested in a closed-loop conveying setup. 
In the future, the Bureau plans to con- 
duct detailed, quantitative studies of 
the impacts that generate conveyor noise. 
It is believed that a better understand- 
ing of these phenomena is needed to 
achieve further conveyor noise reduction. 



Percussion Drilling 

Future Bureau work on percussion drill 
noise will consist of basic studies to 
gain a better understanding of the per- 
cussion drilling process and its rela- 
tionship to noise. Work is planned in 
the following areas: 

Drill Sound-Power Studies 

Sound-power measurements of drills and 
their noise-producing components should 
provide a simple, accurate indication of 
how drill operating parameters (hammer 
pressure, feed thrust, etc.) influence 
the overall noise level. Relationships 
between sound power and drill size, power 
source (pneumatic versus hydraulic) , and 
internal design (valveless versus valved, 
etc.) will be explored. Noise diagnostic 
work is expected to be made much easier 
through these studies. 

Drill Energetics 

Laboratory experiments will be con- 
ducted to determine the relationships 
between noise and blow energy, blow 
frequency, off-centered impacts, bit 
sharpness, etc. 

Drill-Body Noise 

Preliminary studies of hydraulic drills 
indicate that drill-body noise is equal 
to or greater than drill-steel noise. 
The effects of drill-body design and ma- 
terials on drill-body noise levels will 
be investigated. 

Small-Diameter In-the-Hole Drill 

Placing the percussive tool inside the 
drill hose will obviously reduce the 
amount of noise reaching the operator. 
Work is now underway to explore the 
feasibility of extending down-the-hole 
drilling technology to smaller size tools 
(2 in diam or less). 



80 



Alternative Drilling Technologies 



TECHNOLOGY TRANSFER 



At some point in the future, the Bureau 
plans to investigate new drilling tech- 
nologies that would perform the same 
function as percussion drills (i.e., 
drilling small holes in hard rock) in an 
inherently quieter manner. Candidate 
technologies include rotary drilling and 
water jets. 

Perhaps the most promising drill-steel 
noise-control concept now being investi- 
gated by the Bureau is the "concentric 
drill steel." The two-piece concentric 
steel consists of (1) an inner pulse- 
transmission rod to deliver the blow to 
the bit and (2) an outer torque tube to 
rotate the bit and attenuate the noise 
produced by the inner pulse-transmission 
rod. The outer tube is acoustically iso- 
lated from the inner rod by elastomer in- 
serts, and hole-flushing water passes 
through an annulus between the two sec- 
tions. Although the prototype concentric 
steel is being developed under contract 
(39) , much of the testing and evaluation 
will be done in-house, using the pneumat- 
ic and hydraulic jumbo drills. 

Hearing Protectors 

The goal of the Bureau's hearing pro- 
tector research program is to develop a 
reliable, reproducible method of evaluat- 
ing the effectiveness of hearing protec- 
tors worn by miners in the field. The 
previous Bureau efforts discussed in part 
3 showed that much work remains to be 
done in this area. Future Bureau re- 
search will attempt to define (1) the 
theoretical minimum noise level attain- 
able through the use of hearing protec- 
tors, (2) the ability of hearing protec- 
tors to attenuate impact noise, and 
(3) the exact effects of "noise leaks" 
(air gaps, etc.) on hearing protector ef- 
fectiveness. Plans are for MSHA to coop- 
erate closely with the Bureau, and the 
in-house anechoic chamber will be a valu- 
able tool in these investigations. 



Technology transfer is one Bureau 
activity that will continue during the 
shift from short-term contracts to long- 
term in-house research. The only antic- 
ipated difference between past and fu- 
ture practices is that contract final 
reports will be replaced by in-house Re- 
ports of Investigations (RI's) and Infor- 
mation Circulars (IC's) as the primary 
technology-transfer vehicles. As before, 
Bureau personnel will continually review 
new noise-control developments in mining 
and other industries and apply them to 
in-house programs. 

In addition, information concerning the 
mining equipment noise-control technol- 
ogy described in part 3 will be supplied 
to the industry in a more usable form. 
Although the contract final reports cov- 
ering the projects described in part 3 
contain substantial amounts of informa- 
tion, most are much too detailed for 
potential users of the technology. 
Therefore, the Bureau has published a 
handbook (2^) covering all its mining 
machinery noise-control programs in a 
clear and concise format. Figures 66 and 
67 have been taken from this handbook; 
they summarize the current noise-control 
technology for underground mining equip- 
ment (fig. 66) and preparation and 
processing plants (fig. 67). Some of the 
"quieted" noise levels of the machines in 
these figures resulted from Bureau ef- 
forts, some resulted from manufacturers' 
efforts, and others represent the noise 
levels that could be achieved if current- 
ly available noise-control techniques 
were applied to previously untreated 
machines. The Bureau's handbook also 
contains extensive, easy-to-use listings 
that tell how the noise-control treat- 
ments are applied, how noise-control ma- 
terials can be obtained, and where to go 
for further information. 



SUMMARY 



Hearing loss due to noise in the min- 
ing environment is a very serious 



occupational health hazard. The Bureau 
of Mines has addressed this problem by 



81 



KEY 

Unquiet ed 
Quieted 



TYPICAL WORKER EXPOSURE, dBA 
80 90 100 110 120 



Loaders 



^ 



Hand -held pneumatic 
percussion drills 



Jumbo- mounted percussion 
drills 



Rotary face drills 



Cutting machines 



Continuous miners 
(drum type) 



Continuous miners 
(auger type) 



Shuttle cars 



Longwall shearers and plows 



Continuous haulage chain 
conveyors 



Roof bolters 



Diesel-powered load-haul-dump 



^" 



Diesel -powered haulage 
trucks 



D'lesel -powered personnel 
carriers and aux. equipment 



^^^"^^ 



Rail-mounted mantrips and 
locomotives 



'^^B^ 



Face ventilation systems 
(fans and blowers) 



Pneumatic slushers and 
tuggers 



^^^^ 



^^" 




iWp5^-^'N 




^mA. 




80 90 100 110 120 

FIGURE 66. - Quieted versus unquieted noise levels of underground mining equipment. 



82 



KEY 

Unquieted 
Quieted 



TYPICAL WORKER EXPOSURE, dBA 
70 80 90 100 110 



120 




Crushing and breaking equipment 
(jaw and cone crushers) 



Crushing and breaking equipment 
(autogenous grinders and mills) 




FIGURE 67. - Quieted versus unquieted noise fevels of preparation and processing plant 
equipment. 



conducting a wide variety of noise- 
control research programs. During the 
past 10 yr, these programs have involved 
all four major segments of the mining 
industry — underground coal mining, hard- 
rock mining, surface mining, and coal and 
mineral processing. 

Mining machinery is primarily respon- 
sible for the high noise levels in the 
mining workplace. For this reason, and 
beause the workplace cannot be easily 



treated to reduce noise levels, Bureau 
research has focused on noise-control 
treatments for the machines used in 
mines. Retrofit techniques for existing 
equipment have been pursued whenever pos- 
sible; however, in many cases equipment 
redesign has been found to be a more ef- 
fective approach. 

Retrofit treatments have been developed 
for (1) the cutting heads of auger-type 
continuous miners, (2) chain conveyors. 



83 



(3) handheld and jumbo -mounted pneumatic 
percussion drills, (4) dlesel-powered LHD 
vehicles, (5) bulldozers and front-end 
loaders, and (6) coal and taconite pro- 
cessing plants. Redesign efforts have 
been directed toward (1) pneumatic per- 
cussion drills, (2) LHD vehicles, and 
(3) preparation plants. Basic studies 
of coal-cutting dynamics have provided 
valuable information that is now being 
used to design reduced-noise cutting 
heads for drum-type continuous miners and 
longwall shearers. Studies of the effec- 
tiveness of hearing protectors in the 
mining environment are underway. 



The Bureau's emphasis in the past 2 yr 
has shifted from field demonstrations and 
contract research to long-term basic in- 
house research. A reverberation building 
has been constructed to provide Bureau 
personnel with a well-controlled acousti- 
cal environment in which detailed studies 
of the noise-generating mechanisms of 
mining equipment can be conducted. This 
approach is expected to be more cost- 
effective than field demonstrations and 
should yield results that can be imple- 
mented in the future by equipment manu- 
facturers and users to create a quieter 
mining environment. 



REFERENCES 



1. Bartholomae, R. C, J. G. Kovac, 
and J. Robertson. Measuring Noise from 
a Continuous Mining Machine. BuMines IC 
8922, 1983, 17 pp. 

2. Bartholomae, R. C. , and R. P. 
Parker. Mining Machinery Noise Control 
Guidelines, 1983. BuMines Handbook, 
1983, 87 pp. 

3. Becker, R. S. , G. R. Anderson, and 
J. G. Kovac. An Investigation of the 
Mechanics and Noise Associated With Coal 
Cutting. Pres. at ASME Winter Meeting, 
Chicago, IL, Nov. 16-21, 1980. ASME pre- 
print 80-WA/NC-l, 15 pp. 

4. Berger, E. H. Using the NRR to 
Estimate the Real-World Performance of 
Hearing Protectors. Sound and Vib. , Jan. 
1983, pp. 12-18. 

5. Bobick, T. G. , and D. A. Giardino. 
The Noise Environment of the Underground 
Coal Mine. MESA (Dep. Interior) Inf. 
Rep. 1034, 1976, 26 pp. 

6. Bolt, Beranek, and Newman, Inc. 
Bulldozer Noise-Control Manual. Ongoing 
BuMines contract J0177049; for inf., con- 
tact R. C. Bartholomae, TPO, BuMines, 
Pittsburgh, PA. 

7. . Loader Noise-Control Man- 
ual. Ongoing BuMines contract J0395028; 



for inf., contact R. C. Bartholomae, TPO, 
BuMines, Pittsburgh, PA. 

8. Creare Products, Inc. Develop- 
ment of a Prototype Quiet Hard Rock 
Stoper Drill. Ongoing BuMines contract 
H0113034; for inf., contact W. W. Aljoe, 
TPO, BuMines, Pittsburgh, PA. 

9. Daniel, J. H. , J. A. Burks, R. C. 
Bartholomae, R. Madden, and E, E, Ungar. 
The Noise Exposure of Operators of Mobile 
Machines in U.S. Surface Coal Mines. 
BuMines IC 8841, 1981, 24 pp. 

10. Dixon, N. R. Development and 
Evaluation of Noise-Control Techniques 
for Taconite Processing Equipment. On- 
going BuMines contract J0377014 (Bolt, 
Beranek, and Newman, Inc.); for inf., 
contact T. G. Bobick, TPO, BuMines, 
Pittsburgh, PA. 

11. Dixon, N. R. , and M. N. Rubin. 
Development of a Prototype Retrofit 
Noise-Control Treatment for Jumbo Drills 
(contract H0387006, Bolt, Beranek, and 
Newman, Inc.). BuMines OFR 111-83, 1983, 
95 pp.; NTIS PB 83-218800. 

12. Durkin, J. Discriminating Ear- 
muff. Paper in Noise Control. Proceed- 
ings of Bureau of Mines Technology Trans- 
fer Seminar, Pittsburgh, PA, January 22, 
1975. BuMines IC 8686, 1975, pp. 97-108. 



84 



13. Dutta, P. K. Development of Com- 
mercial Quiet Rock Drills. Ongoing Bu- 
Mines contract J0177125 (Creare Products, 
Inc.); for inf., contact W. W. Aljoe, 
TPO, BuMines, Pittsburgh, PA. 

14. Dutta, P. K. , and P. R. Runstad- 
ler. Development of Prototype Quiet 
Jumbo Drills. Ongoing BuMines contract 
H0395025 (Creare Products, Inc.); for 
inf., contact W. W. Aljoe, TPO, BuMines, 
Pittsburgh, PA. 

15. Ferrari, V., and A. G. Galaitsis. 
Integration of Quieting Technology Into 
New Mantrip Vehicles (contract J0199068, 
ESD Corp.). BuMines OFR 62-82, 1982, 162 
pp.; NTIS PB 82-203241. 



22. Huggins , G. G. , R. Madden, and 
B. S. Murray. Noise Control of an Under- 
ground Load-Haul-Dump Machine (contract 
H0262013, Bolt, Beranek, and Newman, 
Inc.). BuMines OFR 78-125, 1978, 76 pp.; 
NTIS PB 288-854. 

23. Industrial Acoustics Co., Inc. 
Taconite Crusher Noise Reduction — Study 
of Acoustical Enclosure for Symons 
7-Foot, Standard Head, Extra-Heavy Duty 
Cone Crusher (contract H0387016) . Bu- 
Mines OFR 82-064, 1982, 33 pp.; NTIS PB 
82-202649. 

24. Lord, H. W. , W. S. Gatley, and 
H, A. Evenson. Noise Control for Engi- 
neers. McGraw-Hill, 1980, p. 331. 



16. Galaitsis, A, G. Noise Reduction 
of Chain Conveyors, Volume II (contract 
H0155113, Bolt, Beranek, and Newman, 
Inc.). BuMines OFR 171-83, 1983, 62 pp.; 
NTIS PB 83-262634. 

17. Galaitsis, A. G. , P. J. Remington, 
and M. M. Myles. Noise Control of a Mine 
Operated Personnel Carrier — Volume I, De- 
sign and Performance of Noise-Control 
Treatments (contract H0166090, Bolt, Ber- 
anek, and Newman, Inc.). BuMines OFR 
133-78, 1978, 112 pp.; NTIS PB 289-711. 

18. Giardino, D. A., T. G. Bobick, and 
L. C. Marraccini. Noise Control of an 
Underground Continuous Miner, Auger-Type. 
MESA (Dep. Interior) Inf. Rep. 1056, 
1977, 57 pp. 

19. Harris, C. M. Handbook of Noise 
Control. McGraw-Hill, 1979, p. 12-10. 



25. National Institute for Occupation- 
al Safety and Health. HEW (now HSS) 
Publ. 76-172, June 1976, 70 pp. 

26. Patterson, W. N. , G. G. Huggins, 
and A. G. Galaitsis. Noise of Diesel- 
Powered Underground Mining Equipment — 
Impact, Prediction, and Control (con- 
tract H0346046, Bolt, Beranek, and 
Newman, Inc.). BuMines OFR 75-058, 1975, 
210 pp.; NTIS PB 243-896. 

27. Pettitt, M. R. Development of a 
Reduced-Noise Auger Miner Cutting Head. 
Ongoing BuMines contract H0188065 (Wyle 
Laboratories); for inf., contact W. W. 
Aljoe, TPO, BuMines, Pittsburgh, PA. 

28. Pettitt, M. R. , and W. W. Aljoe. 
Fabrication Manual for a Reduced-Noise 
Auger Miner Cutting Head. BuMines IC 
8971, 1984, 9 pp. 



20. Hennings , K. Noise Abatement of 
Vibrating Screens Using Non-Metallic 
Decks and Vibration Treatments (contract 
H0387018, Allis-Chalmers Corp). BuMines 
OFR 120-82, 1982, 61 pp.; NTIS PB 82- 
251919. 

21. Hennings, K. , and D. Grant. A 
Simulation Model for Predicting the Per- 
formance of Vibrating Screens (contract 
J0395138, Allis-Chalmers Corp.). BuMines 
OFR 137-83, 1983, 120 pp.; NTIS PB 83- 
238386. 



29. Phillips, W. G. Source Diagnosis 
and Abatement Techniques for Noise 
Control in Taconite Plants (contract 
J0377014, Bolt, Beranek, and Newman, 
Inc.). BuMines OFR 79-079, 1979, 115 pp. 

30. Pokora, R. J., and T. L. Muldoon. 
Demonstration of Noise-Control Techniques 
for the Crushing and Screening of Non- 
Metallic Minerals (contract J0100038, 
Foster-Miller, Inc.). BuMines OFR 50-83, 
1983, 185 pp.; NTIS PB 81-237646. 



85 



31, Roepke, W, W. , D. P. Lindroth, and 
T. A. Myren. Reduction of Dust and Ener- 
gy During Coal Cutting Using Point-Attack 
Bits. BuMines RI 8185, 1976, 53 pp. 

32. Rubin, M. N. Demonstrating the 
Noise Control of a Coal Preparation 
Plant, Volume I: Initial Installation 
and Treatment Evaluation (contract 
HO 155 155, Bolt, Beranek, and Newman, 
Inc.). BuMines OFR 79-104, 1979, 179 
pp.; NTIS PB 299-963. 



33. 



Demonstrating the Noise 



Control of a Coal Preparation Plant, Vol- 
ume II: Long-Term Treatment Evaluation 
(contract HO 155 155, Bolt, Beranek, and 
Newman, Inc.). BuMines OFR 143-83, 1983, 
91 pp.; NTIS PB 83-237354. 

34. . Noise-Control Techniques 

for the Design of Coal Preparation 
Plants (contract J0100018, Roberts and 
Schaefer Co.). BuMines OFR 42-84, 1984, 
135 pp.; NTIS PB 84-166172. 

35. Saperstien, L. W. , and W. W. Kauf- 
man. Audible Warning Signals in Under- 
ground Coal Mines. Trans. Soc. Min. Eng. 
AIME, V. 258, No. 1, pp. 1-7. 

36. Stewart, K. C. , and E. J. Burgi. 
Noise-Attenuating Properties of Earmuffs 
Worn by Miners. Volvime 1: Comparison of 
Earmuff Attenuation as Measured by Psy- 
chophysical and Physical Methods (con- 
tract J0188018, Univ. Pittsburgh). Bu- 
Mines OFR 152(l)-83, 1980, 46 pp.; NTIS 
PB 83-257063. 

37. . Noise-Attenuating Proper- 
ties of Earmuffs Worn by Miners, Volume 



2: Development of a Laboratory Procedure 
for the Physical Measurement of Earmuff 

Attenuation (contract J0188018, Univ, 
Pittsburgh), BuMines OFR 152(2)-83, 
1980, 37 pp,; NTIS PB 83-257071, 

38. Summers, C, R, , and J, N, Murphy. 
Noise Abatement of Pneumatic Rock Drill. 
BuMines RI 7998, 1974, 45 pp. 

39. Technological Enterprises, Inc. 
Development of Concentric Drill Steels 
for Noise Control of Percussion Drills. 
Ongoing BuMines contract J0338022; for 
inf., contact W. W. Aljoe, TPO, BuMines, 
Pittsburgh, PA. 

40. Ungar, E. E. A Census of Mobile 
Machines Used in U.S. Surface Coal Mines 
(contract J0166057, Bolt, Beranek, and 
Newman, Inc.). BuMines OFR 77-78, 1977, 
159 pp.; NTIS PB 284-112. 

41. Walch, R. H. , and G. L. Beech. 
Noise Control of Underground Load-Haul- 
Dump (LHD) Machines. Ongoing BuMines 
contract H0395076 (Eimco Mining and Ma- 
chinery Corp.); for inf., contact T. G. 
Bobick, TPO, BuMines, Pittsburgh, PA. 

42. Wyle Laboratories, Investigation 
and Control of Noise Generated During 
Coal Cutting, Ongoing BuMines contract 
J0387229; for inf,, contact W. W, Aljoe, 
TPO, BuMines, Pittsburgh, PA. 

43. . Noise Control of Longwall 

Mining Systems. Ongoing BuMines contract 
J0188072; for inf., contact W, W. Aljoe, 
TPO, BuMines, Pittsburgh, PA. 



Ti-U.S. CPO: 1985-505-019/20,012 



INT.-BU.OF MINES, PGH., PA. 27881 



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